Microsoft never needed to buy into that; Microsoft set its own standards with MS-DOS long before that, and Windows further cemented Microsoft's position. Where all the UNIXes and UNIX clones were in dire need of standardisation, MS-DOS and Windows were and still are the Microsoft standard.
Even in modern times, POSIX compliance doesn't necessarily mean much when a lot of recent software is being designed Linux-first, with support for other POSIX systems sometimes relegated to third-party or distribution-specific patches. Not to mention all the various implementations of POSIX compliance come with their own little quirks that make things different enough to still require manual checking for this or that.
Additionally, there are plenty of libraries that are compatible with both POSIX-compliant and non-POSIX platforms. Targeting these libraries produces perfectly usable software.
Therefore, POSIX compliance isn't the be-all and end-all of computing. Somehow or other we've managed to muddle through the years without it in Windows, and Nazis aren't riding dinosaurs on blood-soaked streets just yet.
You're making fun of unicorns, but treating 'POSIX' like a magic word.