You don’t need to explain why companies do things but if we’re here it’s worth noting that some companies will burn money to project stability, long term support and to keep options open. They might have some issues but will bank on being a known quality.
Valve is still entirely at mercy of Microsoft which is why they prop up Linux, not because they’re nice or very profitable there. That means they could do a rug-pull the moment it becomes inconvenient. Linux has no stable APIs either and it doesn’t even attempt ABI stability. Linux support hinges on Proton which left unsupported would deteriorate quickly and with no native versions (which are a pain because API/ABI issues mentioned) it could get where Mac is currently rather quickly.
Apple does weird things constantly but other than insistence on Metal (which can be worked around with MoltenVK which Dota2 does) it sounds that Valve thinks their effort, while profitable, is better spent on things that are more profitable. Fair game to them but as a consumer I will voice my grievances.
And as a reminder, my whole point is that it can’t be said that Valve made an effort - it was a one time thing that was quickly left to rot.