Opt out of betas, restart steam! 11GB download, lol.
Why no macOS version?
Non-trivial development costs for a niche market. The website has said there’s no mac version planned for a year or more. Most games don’t bother with mac releases and that trend will only increase now that Apple is firmly decoupling from the PC hardware ecosystem.
If you buy it on Steam, you’d be able to play it through Proton. I assume Proton is available on Macs, but I don’t actually know; please check and don’t hold me accountable.