1) I don't care about reselling games. I've never sold a game or console, and I only buy used games when there is legitimately no other way to get them. I'd prefer to never buy used, and a move to digital makes that easier, as you never have to worry about running out of copies.
2) I hate moving stuff. When I moved a year ago, my books alone were eleven and a half fully packed boxes. Just of books. Each box weighed about 1/4 of what I do. Which means my book collection is nearly three of me. Who wants to deal with that? I've got an iPad now, and am slowly rebuying all of my books. Problem solved, moving means "make sure my iPad is with me" and just redownload the book I want to read. Takes about two minutes. It takes longer than that to figure out which shelf the book you want is on.
3) Convenience. I already mentioned the ease of not having to move all of those things. Another is convenience when it comes to use. Before I stopped buying CDs (about a year and a half ago), my collection could be stacked up taller than I was. If I wanted to mix up my music, it either meant shuffling CDs while driving or ripping the music to my computer and burning yet-more CDs. Now, I have a plug in for my iPad that lets me play my music from there via the tape deck. At some point in the future I will be picking up an iPod and replacing the stereo to integrate it wirelessly. Then all of my music will be available at all times.
4) Replacement. Touched on in 2 again, but a major factor for me. Say my house catches on fire. I'll lose a lot of things that cannot be replaced. My stuffed animals, my dolls, a lot of my clothes, all gone. My games? Steam has them, I'll be redownloading them as soon as I get a new computer. My music? Downloaded again in under an hour thanks to Amazon. Disasters happen every day. People say they like to own things, but it seems to me that you're far more likely to lose your things in a fire than you are to have them taken away by Steam or Apple.
In short, my life is extremely chaotic right now. I could very well pack up and move next week. And a box of game cases or books is just another thing to pack into my car. It isn't worth it. My clothes alone would require more space than I have, to say nothing of things with sentimental value to me. Why waste the space on plastic and paper?