Not hard. But how hard would it be to make large existing codebases which have always been built by VC++ compile against them? Very. Let alone the institutional aversion to admitting their competitors have done something better than them...
Mainly, as an example, I want to be able to have my NodeJS webservice project in the same solution as my MVC website project, and the C# dll client project. The NodeJS solution doesn't need all the assembly folders, or a .Net compile... but it does need the configured prebuild step, that's it. People say to use a "Website" project, but that brings my entire node_modules tree into source control. It's a pain.