There's a pretty big difference between AirPods and AirPods Pro. The former just sort of loosely sit in the outer part of your ear. The latter form a pretty good seal in your ear canal. That's how you get good noise cancelling with Airpods Pro.

The loose fit of the regular AirPods and the wired EarPods never made any sense to me.

This is actually the exact opposite for me. Rubber tipped buds will not stay put in my ears when I move around, while the original airpods models sit within my ears and don't fall out unless I'm doing cartwheels.

This has been my exact experience too.

The AirPods Pro do come with buds in a few different sizes—have you had the opportunity to try both the small and the large types?

I've also heard that the most recent AirPods Pro fit much better for people who have had problems in the past—I think because their rubber buds also have some foam in them, to help them create a better seal.

my $9 wired earbuds from Sony also form a good ear seal by the way. No need to buy the $250 (!) thing from Apple. Unless you don't have a headphone jack.

I've used these to sleep to podcasts or quiet music at music festivals, and they block out the music from outside pretty well. This is because of the flexible rubber seal. My wireless earbuds are hard plastic all the way around and sit (securely) in my earlobes while my wired ones actually go inside my ear canal.

It also depends on the person and the model of Pros, strangely enough. The first generation stayed in my ears perfectly, but the second generation does not.

Yep. The pros also come with a bunch of different silicon ear tips to fit a range of different ear canal sizes.