Because the Democrats like many other left leaning parties, have one tool and every issue is a nail. That hammer is moral outrage and accusing the person of being morally deficient.
It seems to be the only way that they know how to win the argument, but the problem is that they're not winning the argument so it's pointless.
With Trump, they don't know how to fight him on an even keel because they setup Sanders to lose, who was the only leader they had that could actually do this head on. So now they try to take the legs from under him but this cannot work. You're not going to shame people out of voting Trump.
So they're like a robot stuck in a loop, endlessly repeating the same outrage increasing tactic to where it's now too much and they're becoming parody. It also has made Trump look like he's a victim which is the dumbest of all the things they've done.
But there are hundreds of thousands if not millions of conservative Republicans appalled with him, including very prominent ones, including people on his very own staff. He has the lowest worldwide rating of any President ever, and its not close, so there are non-political cross-cultural currents leaning in the exact same direction as what you describe as an "overreaction." Why?
I think it's because most people are either moral or can define their own sense of what is moral. And he is viewed as amoral, if not immoral, not because of influence from "the left", but because those viewers are PEOPLE. That's because of HIS words and HIS actions. They -- we -- are stuck in a loop because HE is. He's accused of being morally deficient because nearly his entire history demonstrates that he is, not because of "the Democrats."
I'm odd in that I think this should be a cause for joy, that so many can coalesce to define and codify "moral", rather than a cry of pain because so many of his supporters cannot.
His supporters who cannot are blind to this because they themselves are amoral/immoral or careless/lazy/stupid (a small fraction, and I have constantly taken umbrage with those who paint all his supporters this way), or they feel abandoned by "the system" (more of them) or because they are using their vote as their last-dash fight-back tool at their own inability to achieve aspiration (more still) or to have their concerns heard (even more), or because he's actually helping them specifically (another small portion).
And they stay with cognitive dissonance -- they have eyes, ears and aren't made of stone, and are in basic agreement with a similar moral code as everyone else who doesn't support him -- because he's still President (the "still" part being a continual victory -- "winning every day" as it were) and because human beings are natural anchorers.
Your reaction to that is "all politicians are in a sense amoral or immoral, so he's an extension of that mold", which, by the way, also could extend to "all PEOPLE are in a sense amoral or immoral." Which maybe they are, but it's a matter of degree, a distinction I think you struggle with because you can't numericize the degree.
What you view as an overreaction I view as your misread of the human condition. That "overreaction" is in my book actually a cause for great hope. That's not meant to be a value judgment on you.