The problem is doing this requires a team to support it that is realistically as large as your average product team. I know Bazel is the solution here but as someone who has used C++, modified build systems and maintained CI for teams for years, I have never gotten it to work for anything more than a toy project.

I have several times built my own system to do just that when it wasn't even my main job. Doesn't take more than a couple of days.

Bazel is certainly not the solution; it's arguably closer to being the problem. The worst build system I have ever seen was Bazel-based.