I think for most projects, contributions are hard enough to come by without having to factor in a new build system.

Honestly even bazel isn't built with bazel. (it can be, but they do that as a test)

It's like rolling a whole new project into your existing ones... For no clear reason.

Perhaps it's a learning experience, however there's no reason to hobble a project with it.

Thankfully, I doubt most established projects would suddenly switch to a custom build system.

Also there are plenty of auxiliary tools developed for build systems which are at best, difficult to reproduce.

If one needs to improve a build system... Try a pull request?