It's not and eventually they will have to find ways to provide POSIX compatibility, it can happen through a separate compatiblity layer or a library. You don't want to break compatibility with millions of lines of already written code if you want widespread adoption. No one is going to rewrite everything from scratch, just because Google says so.
Which "millions of lines of code" are compatible with POSIX without having any Linux or macOS specific code? Or being compatible with Windows for that matter, which is POSIX compliant just in the name?
I think you're hugely overstating the importance of POSIX, not to mention downplaying the fact that POSIX really isn't sufficient requirement to not have to do any code porting.
> Which "millions of lines of code" are compatible with POSIX without having any Linux or macOS specific code?
Those layers would have to be reimplemented to retain compatibility.
> I think you're hugely overstating the importance of POSIX.
People generally understate importance of POSIX, just because it's old. It's impossible to get APIs perfect and you are throwing away decades of work in the name of getting APIs "right". No one is going to rewrite everything from scratch, just because the new APIs look shiny. For reference read about Unix wars[1].
This is caring about developers, just that you don't understand it.