This is true for all products. How much of your closet is filled with stuff you haven’t worn in at least a year. How much of your freezer is full of stuff you’ll never eat. How many of the books on your bookshelf will you actually read before you die?
Physical books (and games) sit there on a shelf and you're occasionally remind you that you bought them and maybe should read/play them. Items in a digital library seem much easier to completely and utterly forget about.
Everyone's different, but at least for me, I don't buy a game with the hope I'll find time to play it sometime in the next month, I buy it when I'm ready to start playing it, like right now.