In practice? Constantly changing hardware with soc vendors that publish nothing and device manufacturers that (have to) keep pushing out new models on a short cycle. Plus many of them have extra shenanigans to keep the bootloader locked so you can’t install a recovery (presuming you had a working one) so you could replace the os. There are some rare exceptions, but the hardware is rare and tedious and not many people can or will work on installable Linux on them.

If you just want to run some Linux userland, there’s ways to do that on top of android, though. Want to get to a Linux like system or run a program? Might be as close as installing a terminal or running adb shell.