What makes you think they didn't elect Harris?
Whenever you install new software professionally, you have a backout plan for the case where it doesn't work. If (when?) Biden becomes too big of an embarrassment, the backout plan is that his doctors find he can no longer function as president. Harris takes over. Any criticism of Harris is both racist and sexist. It’s a complete restart, with a new honeymoon period. The media will gush over the first Black woman president for years. Harris can do no wrong. Any wrong she does will be Trump's, or Biden's, fault. QED (Quite Easily Done)