I can't speak for everyone, but for some of us it has to do with actually having something to own. If we want to, we can loan it, give it or even sell it. If you buy it digitally, you don't own the game, you buy a license to use the game until they decide to not let you access it anymore. Not to mention compatibility issues with digital games on Sony platforms in particular. Want to play X game on your Vita? Too bad it only works on your PSP.
I like to be able to look through the games on my shelf, pick through the cases and look at them front and back. I like to let my friends look at them and suggest games to them, and let them borrow them. When manuals were a thing, the smell of a fresh manual after opening a new game was great. When a package arrives from Amazon, or I go to pick up a game at Gamestop, there is an excitement, or anticipation at finally holding the game I have been waiting for.
I have personally only paid full price for 2 digital only games ever. They both happened to be Nihon Falcom games, and this is no Nihon Falcom game. I will likely pick it up, but it will be when it is on sale 6 months to a year from now for $5 or $7.50.