It’s more like they are scared of Biden.
So much of the Democrats political identity is intertwined with hating Trump that at this point they desperately NEED him to stick around as a boogey man. An impeachment trial will unite the left but spilt the right, take the focus off Biden’s ineffective Corona response and other failures of his administration.
Without Trump and Republicans to fight with, without distracting their followers by blaming Trump and Trumpists for Americas problems they would (1)have to actually deal with some of them (2)allow the spot light to shine on their failure to do so.
That is what this impeachment is about about.