Re: Running a gaming PC without local admin rights is frustrating
Because Valve wants to control your games completely.
I don't know what sort of games you play, but in mine, it's common for my mod list/order, or other aspects of the config, to change. Personally I manage all that through separate, non-Steam software in the form of mod managers and editors (which do, indeed, bring up a UAC prompt every time something changes); but Valve would dearly love to bring it all in house, and have been making tentative moves in that direction for several years now.
And to do that, they want the player to be able to mod the game while it is running - which means, no UAC.