Probably just simple resistance to use of modifier keys in non-technical users, at least on the Windows side. A lot of users never touch a modifier except for Ctrl for copy/paste and maybe Windows for start menu search.
On the Mac side where key combos and modifier use is more widespread among users, it’s probably because there’s no intuitive visual that can be associated with the interaction.
On Windows, I use AltDrag.
Oh, I get having a visual way of doing it with just a mouse for sure. But for power users or even just-a-little-bit-of-knowledge users it's super quick and convenient. When I had to use Windows for work it drove me nuts that the option wasn't there (ended up finding AltDrag thankfully).