I don't know that Biden even really wanted the job when he ran in 2020. The impression that I got was that he was done seeking the presidency after 2016, it seemed like maybe the party convinced him he was the best chance to beat Trump.
I could be wrong, but it's hard for me to believe he wanted to do anything besides retire and enjoy his remaining time with his remaining family.
Once it became clear that he was NOT the best option to beat Trump, he dropped out. It might be as simple as that.
Still major kudos for dropping out either way.
It seems like he is just puffing out his chest to hide his belly. Probably because he knew he was on camera.
Source: have beer belly