And in B2B or B2G you have customers with checklists of features they don't actually need because the person who wrote the checklist is friends with or paid by a particular supplier, or just operating on old information.
That's how you get Windows subsystem for POSIX. Someone in the government had a checklist saying they'd only buy a POSIX compliant operating system, so Microsoft made one. Amusingly, Linux isn't (mostly because who would pay for that certification?)
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.