There's no good reason smartphones couldn't support booting generic OS images today, except that all of the manufacturers want to lock you into their platform. I feel like we're being robbed of the chance to have a "Linux for Smartphones" that just boots on everything and has drivers for all common hardware and none of the vendor added garbage. Stuff like LineageOS is kind of a start, but they're constantly fighting to get a toehold on each new smartphone generation.

That's because the SoC's themselves have almost zero standardization - and quite unlike the custom platforms of the 1980s and 1990s, they aren't even documented. They can barely run with their hacked-together 'board support packages (BSP's)' and factory-installed OS's. Cleaning up the hardware support for even a single one of these platforms so it passes the standards for mainline kernel inclusion and support is a huge amount of effort, and then only gives you long-term support for that one hardware revision. It's a lot worse of a mess than what we had in the 1990s.

Everyone that moved into laptops as their main computer are split between Windows/macOS laptops, with a negligible market size for anything else.