Many things boil down to research I'd have to do to convince myself of feature parity for things important to me. Micro SD cards are mostly gone from Android and also not so important to me as they once we're, so that reason is effectively gone. In no particular order:
1. You're stuck with iMessager, which at least means I can't keep Signal as my default messaging app, and probably means other downsides.
2. Old ipods and my iPad don't seem to even allow file transfer with my computer via USB. I don't know if you can transfer files directly to/from external hard drives.
3. Lightning cables (maybe iPhones don't have these anymore?)
4. I suspect that there are missing features if you don't have a MacBook, but I haven't looked into it.
5. I have apps I know and like. For starters, I would have to find a podcast app that I liked. AFAIK you still can't use FF with ublock origin on iPhones, which means I would have to migrate all my browser activity to something else.
6. Keeping my Android phone at least lets me pretend that I'll migrate over to GrapheneOS like I should.
7. I like having access to FDroid. AFAIK, iPhones have no analogue.
8. It seems like the iPhone mini is missing a fingerprint reader, which means only the SE is appealing.
Anyway, I thought pretty hard about it with my last phone because 2 of 4 Apple phones are the best 2 smaller phones, but it would be a lot of inconvenience just to have a smaller phone.
2. I can sideload whatever I want. Emulators, disallowed apps, etc.
3. Real terminal with ssh, ability to install packages, etc
4. Not locked in to iMessage, iTunes, Apple Music, Safari
5. Some apps that use plugins can only run on Android. Those are right out on iOS.
Not much, apparently. In the US at least, iPhone usage has been steadily eroding Android usage; indeed, as of this summer, the US iPhone userbase became larger than the Android userbase [0].
Also giving more control over the market to company, that is almost definition of walled garden is bad for everyone.
For me by far the number one reason I use my Android device is the Internet browsing differences. Firefox Nightly with all the add-ons I use (first of all uBlock Origin, but so many more) and the sync to desktop Firefox is night and day compared to Safari (with an adblocker).
Overall the current landscape is pretty grim to me. Is it the covid effect we're seeing through poor quality and a lot of stagnation for the last 2 years ?