anaaaaaaaaaaahhh The Steam product pages for Sims 4 and all its packs list the platform as Windows, with no mention of macOS as you see elsewhere. It's very common for games to be supported on multiple platforms in one storefront but not in another, so it's always important to check what you're buying.
Perhaps in the future, Apple will reintroduce Bootcamp or something like it to allow users to partition their drives and install Windows on one side. In the interim, Parallels is a less-than-ideal workaround, both for performance reasons and because you'd have to pay for a subscription.
Another possible approach is Whisky, which is a way to run Windows games in macOS. It's not perfect either, but you might find it worth trying if you have the patience.
