I'd say one of the biggest contributing factors into buying retail product is that people don't always like to buy an entire box of products from online, and may not have an LCS. Almost everywhere has a Target/Walmart/etc. Buying a few packs at the store is like the pack of gum or the 20 oz Pepsi at the checkout line...sure, I'll take a couple of those. Impulse buying. It's true not everybody cares how much ROI you get on a product, but on the flip side, sometimes it's all a collector really cares about. One thing holds pretty true though, and that is that opening packs of cards is kind of like the lottery: most of the time you come out behind, and sometimes you win.
Prizm is one of the nicer retail-available products to hit the market for basketball, so I imagine a lot of people who don't venture into hobby boxes or their LCS often are probably enjoying the opportunity to pick up some nice looking basketball!