Cards add bulk pretty fast, especially for something as flat as the recto verso. If the slots were on the inside, you probably couldnt fit anything else except for maybe 2 bills, some coins, and maybe a receipt? A key fob, if small enough, might be able to fit too.
Some people prefer the card slots on the outside of card holders. This is ideal for people who have to show an ID or use a specific card frequently. College students or people who use public transportation everyday come first to my mind, since that was me in college. If I needed my cards to be more secured, like when going to a bar, I just put my cards inside the zipper compartment of my card holder. RV didn't exist when I was in uni, but I exclusively used slgs like it.
Nothing is stopping anyone from putting their cards inside the flap and zipper compartment, and you don't need slots to "tell you" to. If I'm running an errand for someone else, that's where their card goes for me. Makes it easier for me to keep their receipt with their card.
Personally, I think zip-type wallets are inefficient. I refuse to buy a zippy wallet because I see them as being slow and cumbersome to use. I have to unzip and zip everytime I want something? No thanks. Those few seconds are precious when you're trying to catch a ride. My mom and best friend love zip wallets though, since they see it as being more secure and can carry more things. They travel more often than I do, so they have a greater need for keeping their things together.
Just goes to show that different lifestyles have different priorities, and the things we want to buy and use can reflect that.
