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Short answer: phone manufactures hate you
Long(ish) answer: Each phone manufacture heavy modifies the linux android kernel in order to make it work on there hardware. This usually means that the binary blobs (there are a lot of them) are only compatible with specific kernels.
In addition, phone manufacturers write bad code, that would need to go through many review iterations until the open source community is happy with their changes. Often their changes to the kernel break things for other hardware, for instance.
Review and writing good software takes time, and phone manufactorers don't care about long term support, they want to sell you a new phone every year, or even more often. So they fork the kernel and hack together with lots of string and duct tape, a frankenstein kernel that just works on their hardware, but is broken for anything else.