That's probably because the early access version was compatible with macOS (and natively supported Apple Silicon), but macOS support hasn't been released yet for the full game. It is indeed an unfortunate side effect, but I don't think it's Steam's fault (either Larian's decision to not support the EA macOS version at the same time, or a limitation of Steam of only having one version "published").

I actually asked Larian about this today:

> There isn't a release date yet for the Mac version, but keep an eye out for future community updates!