They aren't acting like that, Pelosi is routinely calling the president and Barr liars. The Senate won't impeach the sitting president so the choice is between having a failed impeachment proceeding that ends in an embarrassing defeat or not starting the whole circus in the first place.
Defeat sure, embarrassment? I'm a little more skeptical about that.
I hate the lazy comparison because the substance is drastically different, but impeachment worked against Clinton.
By the time his term ended his brand was so toxic that Al Gore and Bush were basically running against the image the impeachment had brought him. Gore didn't even really want him campaigning for him. Of course Fox News is really powerful today and such, but that was a guy that
didn't commit a bunch of actual serious felonies.
From a purely ethical standpoint, the spinelessness is even more concerning.