Because the agreement you agree to when you buy their games says you can't. :shrug:
So don't buy Steam games, miss out on many of the really good games, and rail against the machine. But it's here to stay, I don't see it going away anytime soon, it's only been getting stronger and more popular at a rapid rate. I don't resell PC games, never have, and don't consider it any kind of a show-stopper if gaming companies make it so you can't. Sorry for those who want to, but if they're depending on reselling PC games for their primary income... well, there are a lot better and more efficient ways to make some coin. PC games are now one-sale entertainment deals, not resale investment properties. Times change. How you buy games changed. I actually prefer it the way it is now, myself. Since running a used-game resale operation isn't anything I care about, I find the extra services and features and convenience (and really great deals on really great games!) of the whole Steam operation to be a definite bonus. To each their own preferences.
I don't buy their games ~One time, I was forced to use them for this single player game that had no Earthly use for an active internet connection save their Steam DRM ~it had a great heritage, and I thought I must get it (FO:New Vegas). Another two times I "bought"

two games that I thought that I would not be able to find anywhere else ~and had immediate buyer's regret when I did find one of them for sale in a retail store, with the DVD and manuals in the box.
Given the choice I'd not hand them a cent.
PS: sorry if this gives all the rabid DRM/Steam haters an infarction, but you're a dwindling minority... time to adapt or die. ^_^
:Sigh:
This is the same post again, that a lot like to put out in defense of this onerous business practice. Its the new and ain't it great.
Its funny, the military has a crisis of the moment, where they are finding out that the 3D plans for some of their attack subs are interpreted differently by the later versions of the software used to make them ~and its deadly dangerous to trust the new version, and the new methods.
I'll tell you this... in two years I've bought 60 games from GOG, and maybe 7 from other vendors, and I actually have retail versions of several games still wrapped in the box and never installed due to the crap they wrapped them in.
As gamers, games are like food; but when the troff stinks, I tend to look elsewhere to survive, not adapt to the stench of the 'food' additives.
*edited