Many self-described ‘independents’ aren’t actual independents. They are partisans but won’t admit it.

Independents pretty much lean either right or left. As this graph shows Republicans are more likely over the last 25 years to have a more favorable view of anything their party leader does. Some independents who lean right are likely the same way.

Because 'independents' are mostly slightly embarrassed Trump supporters, and all of them politically willfully ignorant.

The less-pessimistic hypothesis is that they were dissatisfied for different reasons under Biden, but yeah, it's not great