Windows zip is not in fact good enough. I've run into weird, buggy behavior, hanging on extract, all sorts of nonsense. I can see the argument that a universally-adopted solution is better, but that's different from windows just not working.
I'm not saying I would ever use it. I'm saying that for casual non-power users, it's good enough. They work with it and if it breaks once in a blue moon they don't care. They just want it to open the files they get and give them a way to send files compressed.
That is enough to bite into 7-Zip's share of users.
