It’s the only OS available for most computers. Sort no one cares about Linux or a BSD. Normal people only know Linux and Mac and you can’t get MacOS with your non-Mac.

And we all know Microsoft caters hard to enterprise and Apple almost seems to give them the middle finger on purpose. So all those big companies only buy MS.

Add in the giant base of existing software, especially line of business/custom stuff, and MS is safe for a long time, no matter how bad they are.