Well, Pete Buttigieg is sharp as a tack, but he is inexperienced and gay.
Elizabeth Warren is one of the smartest people in the Senate, but too liberal and a woman.
Corey Booker was seen as too phony.
Kamala Harris didn't connect with people.
Everyone had problems.
It's not EASY to win an national election. Joe Biden, for all of his flaws, has some really incredible qualities: he's genuine (or seems it), he's completely nonthreatening (which is why none of Trump's attacks worked), he's actually a nice human being-- and that comes across when he speaks, he's non-elitist, he has a shit ton of experience, and he's a photogenic and handsome white male. All of this counts.
And yes, he would have been a better candidate in 2016 than he was in 2020... and he would have been a MUCH better candidate than Hillary was.