Makes sense, because Windows is also used for servers. The Steam client is not.
> If Steam is installed on a corporate network PC (game development, QA services, esports teams, etc?), regular users being able to elevate to machine admin is a big deal.
No it isn't. This is effectively the same as being installed on some home PC. Being local admin on the workstation doesn't really give you any more ability to do damage than just a regular user account. A non-administrative process still has access to all the user's stuff, and all the network resources that user account has access to. Local admin doesn't really give you anything new.
That's dangerously untrue. On all the Windows systems at home, everybody else is a non-privileged user and I set sane file permissions so they can get to shared movies, etc, but not read my bank details and tax records. Privilege escalation means that they could trivially change those permissions. The same is true of a corporate environment -- just because you can't also make changes to AD doesn't mean the threat of accessing local files under other users' accounts is trivial.
ETA: I just read a bunch of this user's other posts in this thread and I'm beginning to suspect they're trolling, so I'll disengage.
Ok, in that scenario it is true, but multi-user desktops in the age of smartphones and tablets are a vanishingly small niche. My experience in corporate environments suggests they are also almost entirely 1:1 as well.