Giving up power is way more respectable than trying to hold onto power. Something Biden’s opponent, now Harris’s opponent isn’t good at.
There’s a lot of people that are old and don’t follow politics all that closely. Biden’s story of having to accept that he’s gotten too old to do the things he wants to do is a story they can relate to. The guy they can relate to most is endorsing Harris. It’s not like he’s gone. He can still make speeches about how the GOP wants to cut Social Security and his friend Kamala won’t do that.