And then...
Why can't Linux run Windows programs?

Is the kernel the issue here?
Couldn't the kernel coders allow some "emulation" of some other OS's system calls so that the executable that runs on that OS runs just as fast on the one with the "emulation"?

The big question I'm asking is:
Could Windows add functionality to itself, to run Linux binary executables (ELF) efficiently and natively, if they really wanted to?
Is it possible?