The issue is that, while the CPU instruction set is largely (completely?) compatible between systems, the peripherals are not, and the drivers are often handled by closed-source binary blobs that are not portable to other operating systems. So while you could get code to run on the CPU, you wouldn’t have networking, display, audio, etc. Same reason you can’t just drop Linux on any old Android phone or tablet either (some you can, but not many).
The issue is that, while the CPU instruction set is largely (completely?) compatible between systems, the peripherals are not, and the drivers are often handled by closed-source binary blobs that are not portable to other operating systems. So while you could get code to run on the CPU, you wouldn’t have networking, display, audio, etc. Same reason you can’t just drop Linux on any old Android phone or tablet either (some you can, but not many).