You can have ~/.config/. Nothing in macOS prevents you from having it. And so, some programs do. The worst thing that happens is that, instead of having one directoy ~/.foo you now have one directory ~/.config/foo and nothing else in ~/.config. But as soon as you add the second thing that uses ~/.config, you now have two directories in there instead of a second dotdirectory in ~.
It's just that for a bunch of them the XDG path is only used if it exists - e.g. emacs predates the spec, so it uses ~/.emacs.d (and a few others) first.
Cargo doesn't use the XDG paths at all, apparently - https://github.com/rust-lang/cargo/issues/1734. However it also needs a directory for binaries (~/.cargo/bin) and ~/.local/bin isn't actually in the spec at the moment (https://gitlab.freedesktop.org/xdg/xdg-specs/-/issues/14).
Nobody stops Apple developers respecting a Freedesktop spec, but the point is many people that mostly know macOS probably didn't even know XDG was a thing. It's not like Apple encourages it in any of their command line utilities.