If there were no evidence, the investigations wouldn’t be continuing. It’s not normal for FBI or Intelligence investigations to disclose evidence before they’ve reached a conclusion, so it often looks like they have nothing. In the meantime, you have to remember things that are not private: like the fact that Jeff Sessions perjured himself during testimony on 3 separate occasions, or the fact that a growing number of Trump’s administration are being forced to come out as agents for foreign powers, forced to resign, or are getting fired. The turnover in this administration is worse than Taco Bell. These things lead to the old duck theory: If it looks like a duck and sounds like a duck it must be a duck, but until we find a lost tailfeather it could be a guy with a decoy distracting us from the endangered species he’s killing.
Also, the end goal of the investigations isn’t necessarily just Trump. There’s a good chance they are working on proving a case for delegitimizing the entire election, which will take a lot more evidence than what they might need to take down one man.