Because you're completely, completely, not understanding how expensive Vita cartridges are. If this was just PS4, sure, but it isn't. I'll say it again, Vita cartridges are expensive. We don't make much money on Vita games and we likely never will. If anything we'll make less money as time goes on and costs for the media rise. Rather than charge $50 on Vita and $30 on PS4 - I'd just prefer consistency and parity between the platforms in terms of cost and uniformity.
If we're selling two Vita cartridges for essentially the price of one - we might actually end up making negative money. This is the only financially viable approach on Vita. I realize that the Japanese Gundam game (which is $50) has two Vita cartridges, but keep in mind Namco and a few other big Japanese companies own their own factories for game production so they don't pay nearly as much as the little guys.
The cost of Vita carts is why I can't help but laugh when I hear that people are shocked by how much Switch carts cost to manufacture. It's nothing new - cartridges aren't cheap.
