Building on standards like POSIX prevents vendor lock-in, which is beneficial in the long run because it prevents the vendor from holding you hostage once you start relying on the system. It's a sensible requirement.

Microsoft's deliberately useless POSIX support is a result of Microsoft acting in bad faith and sabotaging the efforts, as usual, because the lock-in is what they want. Just like they did with OpenDocument, for example. And what they tried to do with Java and the web.

It's also because POSIX is completely useless.