Why would they even call for an investigation if they didn't have enough evidence?
That's an odd question, isn't it. Because of a reasonable suspicion founded in evidence. Like any other situation.
Why do conservatives seem so often to not understand basic things? Is this really so hard to figure out? You don't start an investigation because you have a certainty. You start one because you have doubts.
Even though the DOJ and the FBI were spying on the Trump campaign they still have found no evidence.
That's just not true at face value. You can't call even what's publicly known "no evidence". You can bet Mueller has more. And as far as I'm aware, the FBI was surveilling Manafort, pursuant to a FISA court warrant on an unrelated matter, when Trump decided to hire him as his campaign manager (just bad luck, I guess), and Surgey Kislyak, a Russian diplomat/spy with no legal protections against US spying. That's how they got caught. Monitoring Kislyak lead them to Flynn, and suggested a larger concern, and now the scandal threatens to consume the whole administration. And then separately, Kushner lying repeatedly on his security clearance forms implicates him and could spread further.
They have given up on that and are now trying to find evidence of obstruction of justice.
Obstruction of justice is an independent crime, and why would Trump people be obstructing an investigation in which they were innocent? But all indications are that the investigation on both conspiracy and obstruction charges is continuing.
And the indictments had nothing to do with any Trump Russia collusion.
So far. Those investigations also started before the Trump campaign even began. And they have suggested that superseding indictments are expected to include additional charges.
The woodchipper of justice isn't always fast, but once it locks onto you, it's only a matter of time.
If they were really looking for anyone obstructing justice a case of that is right before their eyes. Thirty thousands of Hillary's emails were destroyed to prevent them from being subpoenaed. That amounts to obstruction of justice.
That was investigated. Not only could they not find evidence to support the allegation that the emails were deliberately destroyed, but they also found positive evidence that something else happened. Would you like to know what? Have you been paying attention? Or are you only interested for as long as it gives you a cudgel to attack Hillary Clinton with?