The problem is that even if somebody out there is willing to go to the work of making device drives, bootloaders and OS releases for the hardware, it still can't happen because the information about how to talk to the hardware isn't open, same thing for the bootloader and even if those things are reverse engineered often even the ability to install an OS there is locked down on the hardware.

As you correctly say, the dream of any software just running anywhere isn't possible because that's not how hardware works, but the current situation is not one were what's blocking it isn't just the natural architectural structure of hardware, it's one where the hardware makers have purposefully and to quite an extreme level locked down their hardware so that even if people are willing to do the work of doing what it takes to run an OS there, they can't because necessary info to use the hardware isn't available and the hardware is even locked down against OS installations for those who don't have the necessary cryptographic keys.