Custom roms would be more popular if every app dev and Google weren't doing everything in their power to make their software not work on custom roms.
That's intentional. It didn't used to be that way.
2. Bootloader locking matters because many (most?) apps will just refuse to run.
3. Why? Because Google built up the attestation API which severely handicapped those 99.9% of devices from running custom roms.
4. Even on the 0.1%, the Google pixel, it won't pass full attestation - so some really picky apps like banking won't run. Only stock android untouched will run those apps.
It didn't use to be this way 10 years ago.
Most Android users choose that ecosystem due to the price point, as most of the world can not afford iPhones (even second hand ones).
Only a tiny fraction of the billions of Android users out there, chose it for its more open aspects.