Because nowadays Linux ABI is more relevant than pure POSIX compliance.

BSDs and all surviving mainframes and micros also have either Linux VM support or syscall compatibility.

Microsoft saw those geeks that were buying Apple as shinny Linux, and then complaining about lack of compatibility, or that they only cared about POSIX toys but not really Linux proper, and saw a business case in shipping Linux in the box to counter that.

Which had they kept and improved POSIX support since Windows NT 3.51, that would never been a matter to discuss about.