AirPods are held on ears similar to how a wine glass should be held in hand. There is a opposing pair of protrusions in human ears, between which the stem goes through, and the flexion of that protrusions works against the force exerted by the bulb and hold earphones in place.

I don't understand why Apple went with that as a default; that part of my ears seems to have evolved so as not to catch debris, and EarPods-style earphones just comes off as they should. And it is not that likely I have a million in one ear leaf genetic subtypes.