> I just don't get how someone could have at any point said "yes, this is a good path, we should continue with this" for all these custom build systems in the wild on these projects.

I guess nobody ever said that. What people most likely were saying was: "This is a mess, but the expected costs of continuing this path will probably not be greater than the expected costs of changing course".

Note that there's a double "expected" in there. Making decisions under uncertainty is hard. On the other hand, taking a look from the outside and replacing every "expected costs" with whatever turned out to be the case is easy.