Imagine if there were multiple incompatible and competing linux kernels. What we have now is AMD/MS/Apple etc... contributing to the kernel through "vendor code". Imagine if AMD released a AMDLinux and Nvidia had NvidiaLinux.
This already happens, because most (?) people aren’t running vanilla kernels. Many (most?) distros compile their kernels with config options and patches that “make sense to them.” In the most egregious cases, you end up with things like bpf being intentionally broken by default.