Why? The point's clear, either Democrats are letting Trump off the hook for being the complete and utterly contemptuous shitbag we literally all know he is, or that Trump turned out to be a more or less thoroughly mediocre president who behaved exactly as expected of the office in terms of ethics and policy, and Democrats have been talking out their fartboxes for four years with no real intent of doing anything about it.
The only dispute I take with the statement, is both are simultaneously true. Trump behaved exactly as any contemporary president can be expected to behave, like a complete and utterly contemptuous shitbag. His mortal sin was making no effort to hide being a complete and utterly contemptuous shitbag; rather, he seemed to put great effort into and derive much pleasure from making it as visible as possible. And as can be expected from the opposition party, they're going to let him walk because to do otherwise means future Democratic presidents can and would be held liable for being complete and utterly contemptuous shitbags.
All the Democratic party has engaged in for four years, has been kayfabe.
This is exactly why and how impeachment played out as it did. Democrats ran in 2018 on "investigate and impeach", and gave themselves just enough rope to hang themselves with, because Democrats were then expected to follow through. Which is why Pelosi sat for a whole-ass year on it despite in that time the Mueller report having been released, that is until Trump started in on Hunter Biden which was an actual scandal that could have fatally damaged the candidacy of basically the only person in the Democratic field capable of beating Bernie.
Congressional Democrats knew impeachment was a non-starter to begin with; they didn't have enough on Trump to make it happen, and even if they did, Senate Republicans would simply vote to acquit regardless of material fact. And, they knew damned good and well that came with a very real, very likely, chance of political blowback because they saw what happened to Republicans in '98 after impeaching Clinton. What forced their hand, was the need to protect Joe Biden's presidential bid.