Pelosi has no interest in good governance, let alone such trivial things as national unity, patriotism and comity.
Her focus is solely on winning elections, and knows there is no more potent means for rousing Dem voters to the polls, especially those who don't normally vote, than the spectre of Donald Trump.
Impeachment is the best means for Pelosi to keep picking at the scab that drives Dem turnout and fundraising. It is utterly cynical and selfish.
And Joe Biden lacks the balls to stand up to her.