Further, Biden dropping out would cause another civil war within the Democratic Party that is already dealing with one between the pro-Hamas and pro-Israel sides. Kamala Harris is so unpopular that she absolutely cannot win, but parachuting in Gavin Newsom, who has his own handicaps,[1] would cause Harris's side to cry racism/sexism/etc. And this is all setting aside the difficulty of whoever is Biden's replacement campaigning to become president against someone who has, as you said, already done the job for four years. Four years which, polls consistently show, more voters look back positively upon than not.

[1] The best description I've heard of Newsom is that he looks like the mayor of Gotham City.