It's legacy of IBM PC compatible standard, that has multiple vendors building computers, peripherals that work with each other. Microsoft tried their EEE approach with ACPI that made suspend flaky in linux in early years.

This does not explain why the drivers for all the hardware is upstreamed almost immediately in the x86 world but remains locked away in vendor trees for years or forever in the ARM world. Vendor kernels don't exist due to the lack of standardized boot and runtime discovery.