I'm wondering if it is just me. I like looking at my collection of PC games on a shelf in my book case. Also, I am not privy to and don't want a ? terabyte hard drive to put tons of crap on. I have known people that brag about all of the stuff that they have downloaded. They will say something like I downloaded x gigs of content over the weekend. I say what kind of content? They say just miscellaneous stuff . I say why. Just because it is available doesn't mean it should be on my computer. Give me the game on a disc any day unless the DL is the only way to get it.
I'd much rather have a game take up digital space than physical space. $150 for a 3TB+ HDD is worth it compared to a bookshelf that is properly secured, not to mention physical media often costs more, and downloads are required anyway because of patches and extra content. And even if I couldn't store every game I have a license to on my HDD, it's hosted on some company's server (probably Valve's) that I could download/install any time I want, not to mention if I was ever so inclined, I could make my own physical back ups for installation on the medium of my choosing (storage server, CD, DVD, BD, USB Flash, external HDD, etc). Installing disc based media is not the business - typing in absurdly long product keys and constantly switching discs, especially if it's The Sims, is a real hassle, whereas a digital install I just click and let it do its thing in the background while I watch some Netflix, fap, or whatever I feel like without interruption. It's also a major pain to insert the disc into some noisy optical drive just to launch the game. And if the disc gets damaged, well you're just SoL, unless you're willing to download some sketchy no-CD crack from some shady website.
Long story short: it's just you. And just because it is available doesn't mean it should NOT be your computer; The question should never be why, but why not? Personally, I like the convenience of launching a game whenever I want without having to fetch and load physical media.