Same here. It's, obviously, ear shape. I had some in-ear monitors made a few years ago and they take a mould of your ear so they fit perfectly. The woman that fitted them told me I had funny-shaped ears. The standard Apple Airpods won't stay in my ears if I'm just sitting still and concentrating on them not falling out and the sound, for me, is not good
Eventually I bought some Beats Fit Pro that have the springs that sit inside the ear and they work OK for my funny shaped ears, although I did have to experiment with a few aftermarket tips to get a good seal for noise cancelling - again, due to my annoyingly mis-shaped ears - but they don't fall out and I can run with them.
Before that I bought a variety of things that fitted over the standard wired Apple buds that used to come with iPhones and I managed to find some that worked very well for me. The problem is that although might fit some Airpods, they're a pain because they won't fit in the case but if you're interested then Earhooks and Yurbuds were the ones I found to be the best for me.