I believe the difference is that the Linux kernel itself is not that big a project. Windows is a lot more than just a kernel; it’s more akin to having the source code for an entire Linux distribution in one repo.
Some parts of Windows have split out into their own repos and CI build and test pipelines, but splitting is a process that takes people's time and has pros and cons, especially when changes in the main OS repo are needed to give the separated components a complete, coherent API surface.