> Again, only because the platforms are locked down and there is no single standardized target for universally bootable ROMs in the way a Linux ISO can boot on any PC.

Couple of things here. Android was never that open to begin with. There were always (and still going) issues with the kernel sources being withheld indefinitely, AOSP side changes are almost never released at all, iirc only Google released anything. Bootloaders were always an issue, not all manufacturers had unlockable (or easily unlockable) bootloaders.

Ironically, Google were always the spearhead of the standartization in the Android world. Project Treble and stuff resulted in the fact that almost any Android device released these days can run a generic Android image, it's not gonna be as functional as the original Android, but it's something.

Yes, I am on the younger side, that's why I know how it was in the Android world. Nobody is interested in reinstalling OS-es on their phones because the amount of effort to create an Android ISO that's as usable as what's preinstalled is truly gigantic. Android flavours are heavily customized for the needs of the OEM, and for the hardware that they're gonna run on.

It's a niche, always was and always will be. A niche powered by a few large players like LOS or GOS, that actually get paid somehow or heavily work for the idea, and an army of teenage unpaid maintainers, who quit the maintainership as soon as they get a properly paying job that takes most of their time (me).