We wanted the option for distro maintainers to include .NET in the native distro package feeds. This means that distro maintainers have to build the product, not us, and that means creating a build system that meets their requirements. So you either end up with two build systems, or you try and unify. The only direction that it's feasible to go is towards the Linux distro model. It's the most restrictive.
The good news is that the distro model is SIMPLER. It may not be the most performant. A really good distributed system with caching would be far faster. But that's not a solution that's easy to implement or compatible with distro maintainer workflows. Optimizing for simpler in this case is better though. We want the community to be able to participate in a meaningful way. Build for BSD, build for S390x, build and include in distro feeds, etc. We can't feasibly support every platform and scenario that the community wants.