- Unlock bootloader (depending on vendor, you have to do an online verification),
- flash a recovery.img,
- load into recovery mode (which, depending on the phone, might need extra work)
- wipe some caches,
- select new os/rom image,
- pray it doesn’t brick your phone.
You’d think someone would’ve learned a thing or two from the easy graphical installations linux and even windows have been offering since the late 2000s.
I’d say that happens mostly because they don’t even know there are alternatives. Also, like machinin said earlier, bloat is a very common problem in Android phones, even high end stuff from big companies, Samsung being one of the worst offenders in that regard.
The average user doesn’t know the phone doesn’t need half of that shit, so he just shrugs and carries on.