It makes sense to port their application software to other platforms so more people can use it. But what would they get out of giving their competitors the abi

It makes sense to port their application software to other platforms so more people can use it. But what would they get out of giving their competitors the ability to run non-Microsoft products made for Windows? That is a big, non-trivial task which they'd be spending a lot of engineering effort on, for free, and wouldn't benefit them in any way. It would strictly reduce the number of reasons to use their products.