Because here's the thing: the evidence of collusion between the Trump campaign and the Russian government over the election in 2016 is merely suggestive. It's flimsy. It would never stand up in court. At least nothing that's been publicly revealed.
But what is true is a non-stop series of lies and prevarications from Trump and his team over contacts with Russia, or actions taken regarding Russia. None of them are big lies, or deal-breaking lies, or smoking-gun lies. What they are is intriguing lies, because there's little reason to make them.
"Oh, I never contacted the Russians, never, ever ever." Except that it keeps turning out they did — quite possibly innocuously, but by lying about it or "forgetting" about it, or , it just raises suspicions, over and over and over.
"No, none of us in any way shape or form ever influenced the platform committee about providing arms to Ukraine." Except more and more people keep providing evidence that they did, all the way from the top. It's a little action that, by Team Trump's secrecy and then its lies keep making it a bigger and bigger deal.
They act guilty. Even if pure as the driven snow, they are acting in a fashion that is almost calculated to make them look guilty as sin, and in a way that even a full exoneration after an open and public investigation would never make fully believable.
Trump’s Untruths About Russia Are Piling Up – The Atlantic
The need for Congress to figure out why he and his team keep misleading the public about Russia grows more urgent by the day, even if they are ultimately exonerated.
What was it they impeached Bill Clinton over again … oh yeah … they because he lied about something. :-/