But they do. There are very few Christian ecclesial communities who reject Easter---that the Lord Jesus is risen---as fictional.
It began in AD 1054 when many eastern bishops rejected the successorship of the Apostle Peter's supreme pastorship of the Church. But that schism only resulted in a small splintering between the handful of Orthodox churches and the Catholic Church. The Reformation was a further rejection of authority, this time, a rejection of all bishops. No Christian traditions stemming from the Reformation have any bishops at all. They are all pastored by what effectively are deacons.
So bottom line, it's because so many Christians have rejected the Church's overseers/elders, which the Lord and His Apostles set in place in the first century.
Sorry Nihilo but it started way before 1054.
Just as prophesied by Paul here....
29:“For I know this, that after my departing shall grievous wolves enter in among you, not sparing the flock.And of your own selves shall men arise, speaking perverse things, to draw away disciples after them.”.
THE FIRST EPISTLE OF CLEMENT TO THE CORINTHIANS
St. Clement of Rome
Chap. I. The salutation. Praise of the Corinthians before the breaking forth of schism among them.
THE Church of God which sojourns at Rome, to the Church of God sojourning at Corinth, to them that are called and sanctified by the will of God, through our Lord Jesus Christ: Grace unto you, and peace, from Almighty God through Jesus Christ, be multiplied.
Owing, dear brethren, to the sudden and successive calamitous events which have happened to ourselves, we feel that we have been somewhat tardy in turning our attention to the points respecting which you consulted us;1
and especially to that shameful and detestable sedition, utterly abhorrent to the elect of God, which a few rash and self-confident persons have kindled to such a pitch of frenzy, that your venerable and illustrious name, worthy to be universally loved, has suffered grievous injury. For who ever dwelt even for a short time among you, and did not find your faith to be as fruitful of virtue as it was firmly established? Who did not admire the sobriety and moderation of your godliness in Christ? Who did not proclaim the magnificence of your habitual hospitality? And who did not rejoice over your perfect and well-grounded knowledge? For ye did all things without respect of persons, and walked in the command-merits of God, being obedient to those who had the rule over you, and giving all fitting honour to the presbyters among you. Ye enjoined young men to be of a sober and serious mind; ye instructed your wives to do all things with a blameless, becoming, and pure conscience, loving their husbands as in duty bound; and ye taught them that, living in the rule of obedience, they should manage their household affairs becomingly, and be in every respect marked by discretion.
Chap. II. Praise of the Corinthians continued.
Moreover, ye were all distinguished by humility, and were in no respect puffed up with pride, but yielded obedience rather than extorted it, and were more willing to give than to receive? Content with the provision which God had made for you, and carefully attending to His words, ye were inwardly filled with His doctrine, and His sufferings were before your eyes. Thus a profound and abundant peace was given to you all, and ye had an insatiable desire for doing good, while a full outpouring of the Holy Spirit was upon you all. Full of holy designs, ye did, with true earnestness of mind and a godly confidence, stretch forth your hands to God Almighty, beseeching Him to be merciful unto you, if ye had been guilty of any involuntary transgression. Day and night ye were anxious for the whole brotherhood, that the number of God's elect might be saved with mercy and a good conscience. Ye were sincere and uncorrupted, and forgetful of injuries between one another. Every kind of faction and schism was abominable in your sight. Ye mourned over the transgressions of your neighhours: their deficiencies you deemed your own. Ye never grudged any act of kindness, being "ready to every good work." Adorned by a thoroughly virtuous and religious life, ye did all things in the fear of God. The commandments and ordinances of the Lord were written upon the tablets of your hearts.
Chap. III. The sad state of the Corinthian church after sedition arose in it from envy and emulation.
Every kind of honour and happiness was bestowed upon you, and then was fulfilled that which is written, "My beloved did eat and drink, and was enlarged and became fat, and kicked." Hence flowed emulation and envy, strife and sedition, persecution and disorder, war and captivity. So the worthless rose up against the honoured, those of no reputation against such as were renowned, the foolish against the wise, the young against those advanced in years. For this reason righteousness and peace are now far departed from you, inasmuch as every one abandons the fear of God, and is become blind in His faith, neither walks in the ordinances of His appointment, nor acts a part becoming a Christian, but walks after his own wicked lusts, resuming the practice of an unrighteous and ungodly envy, by which death itself entered into the world.
Chap. IV. Many evils have already flowed from this source in ancient times.
For thus it is written: "And it came to pass after certain days, that Cain brought of the fruits of the earth a sacrifice unto God; and Abel also brought of the firstlings of his sheep, and of the fat thereof. And God had respect to Abel and to his offerings, but Cain and his sacrifices He did not regard. And Cain was deeply grieved, and his countenance fell. And God said to Cain, Why art thou grieved, and why is try countenance fallen? If thou offerest rightly, but dost not divide rightly, hast thou not sinned? Be at peace: thine offering returns to thyself, and thou shalt again possess it. And Cain said to Abel his brother, Let us go into the field. And it came to pass, while they were in the field, that Cain rose up against Abel his brother, and slew him." Ye see, brethren, how envy and jealousy led to the murder of a brother. Through envy, also, our father Jacob fled from the face of Esau his brother. Envy made Joseph be persecuted unto death, and to come into bondage. Envy compelled Moses to flee from the face of Pharaoh king of Egypt, when he heard these words from his fellow-countryman, "Who made thee a judge or a ruler over us? wilt thou kill me, as thou didst kill the Egyptian yesterday?" On account of envy, Aaron and Miriam had to make their abode without the camp. Envy brought down Dathan and Abiram alive to Hades, through the sedition which they excited against God's servant Moses. Through envy, David underwent the hatred not only of foreigners, but was also persecuted by Saul king of Israel.
Chap. V. No less evils have arisen from the same source in the most recent times. The martyrdom of Peter and Paul.
But not to dwell upon ancient examples, let us come to the most recent spiritual heroes. Let us take the noble examples furnished in our own generation. Through envy and jealousy, the greatest and most righteous pillars[of the Church] have been persecuted and put to death. Let us set before our eyes the illustrious apostles. Peter, through unrighteous envy, endured not one or two, but numerous labours and when he had at length suffered martyrdom, departed to the place of glory due to him. Owing to envy, Paul also obtained the reward of patient endurance, after being seven times thrown into captivity, compelled to flee, and stoned. After preaching both in the east and west, he gained the illustrious reputation due to his faith, having taught righteousness to the whole world, and come to the extreme limit of the west, and suffered martyrdom under the prefects. Thus was he removed from the world, and went into the holy place, having proved himself a striking example of patience.