Incorrect, you are not buying the files or even the disc they may come on (though obviously, some of that is factored into the cost), you are explicitly buying a license to play the game. The method of distribution just depends on what is most effective. It has worked this way pretty much since Bill Gates introduced licensing to the software world. The only difference is that with online activation, the rights holders have more power to de-authorize licenses that they deem invalid.
You seem to be a bit confused about how software licensing works (and I'm not just referring to Steam). I'd recommend reading any EULA, including older ones (before the age of digital distribution), and you'll see what I mean.
