Because we need a primary to vet candidates. The bad shit comes out in the primary. We weed out the people who seemed great on the surface yet ended up having issues. Just ask leading Republican Presidential candidate Rhonda Santis.

This late in the game there are also a number of non-political things that would go very badly if we switched candidates. It's too late from an administrative perspective. A replacement candidate can't create and launch a whole new Presidential campaign in a matter of days.

Then there's funding. Biden can't just give all his campaign funds to some other candidate. People donated that money to Biden's campaign; there are rules on how that money can be spent.

These are just a few examples of many issues.

It would be an absolute mess in ways that most people have not fully considered.

Then the Republicans would spin the scramble for a new candidate as incompetence of the party as a whole - a rare instance of them making a good point. No way voter confidence increases once all factors have come into play.