Others have touched on the political machinations aspect.

But a bigger, arguably “real”, reason, is that replacing Feinstein is largely trivial. Democrat from a (kind of) Democratic state. But getting said replacement approved on various committees, some of which are republican controlled, becomes the big mess. Which leads to more dysfunction and power stealing and less ability to even discuss if a vote should occur on something.

It is similar to the RBG issue. She should have stepped down YEARS before she died. She didn’t. And by the time people realized “yo, your ass is old and kind of stupid” it was too late because republicans would have just stalled until they could replace her with someone they wanted… which they did.