Linux kernel API itself isn't really enough to do eg. any UI software, because things like creating windows or showing notifications, are not part of it directl

Linux kernel API itself isn't really enough to do eg. any UI software, because things like creating windows or showing notifications, are not part of it directly. You need additional software on top of that for those features. Win32 API is much larger than Linux kernel API and is stable so there is quite large chance that a couple decades old software still works without recompiling or updating libraries. It has its issues with many parts of it being completely horrifying to use, but the stability is something that Microsoft has done well.