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.