The appendix was/is a repository for gut biome when humans getting diarrhea was as regular as Tuesdays. An occasional death from a burst appendix was the lesser evil
It was really important back then. But then we developed safer food/water practices and it was basically useless. But still bursting on occasion.
So there was a very slow march towards people just not having them. Over a long enough timeline that would have spread and the appendix would have gone the way of the human tail.
But now that we can remove them, there's no negative evolutionary pressure, and we're probably going to stay around .001% of people born without an appendix.
It's a great example of how medicine and science actively stalls evolution.