In what way?
The binary formats are not compatible, not even the format of the files themselves. Linux uses ELF. MacOS uses MachO.
True, macOS is more or less POSIX at the base but the API Mac applications are written to is not that at all ( Cocoa ). GNUstep exists for a reason. Sadly, it is not very mature. It is certainly not a trivial undertaking though as there have been a number of attempts over decades and nobody has really pulled it off.
The Win32 API on the other hand has largely been implanted on Linux. A few Win32 APIs are even being added to the kernel.
Going the other way is easier. You can port POSIX stuff to macOS fairly easily.
Absolutely.
A lot of it is just the organization and leadership within the projects themselves. The GNUstep guys struggled for a long time. Just agreeing to implement the Mac APIs instead of just the NeXTstep ones is a thing.
Regardless of how attractive projects are, they can be run well or badly. Without trying to disparage anybody, look at the progress of WINE vs ReactOS for example. And if you think it is just because kernels are hard, look at Linux or Haiku or SerenityOS vs ReactOS instead.
But the popularity of Windows made the Win32 APIs more commercially viable as well and so you get companies like CodeWeavers and Valve that really accelerate the WINE effort. That wind at your back really helps.