I’m sorry if this sounds like a stupid question but why won’t android just straight up run like Linux distros that almost run on any computer? Why does each model under each manufacturers require separate maintainors seperately? Aren’t they running like almost similar hardware like processors from Qualcomm, mediatek, Exynos, etc? Why hasn’t there been a single android os that is compatible with all the device?
The hardware is generally not open source, and it’s technically illegal in a lot of jurisdictions to redistribute the binary blobs that make it work without permission from the manufacturer (basically never has happened). Because there is minimal focus on forward and backward compatibility, different chips from the same product line are very often incompatible with each other, let alone from different manufacturers.
So phone ROM maintainers have the job of stripping out those binary bits from a recovery image for a given phone or something and then getting them working with an open source build of android. All the while taking a (albeit kinda low) legal risk. Take a look at most of the long running open source Android projects and you’ll see they have had to structure their projects to mitigate legal exposure.
In Linux most of the hardware support is just straight up open source, the bits that aren’t (typically GPU or specialist hardware drivers) are manufactured by people that want you to use their hardware on Linux, so they release prepackaged drivers for you to use.
Phone manufacturers are motivated to try and ensure everyone uses their stock ROM because ultimately they have to support the device if a user breaks something, and that’s easier for them when they eliminate as many variables as they can. Even if they can charge the user out of warranty, it’s still tying up support resources from their perspective.
They also make deals with companies as to what their preload has, what the preload forbids from being uninstalled, and what subscription services for into their platform.