It's not necessarily difficult, and it's not prohibited, it's just not very profitable. The problem is you're cutting out the whales. You might have a $5 gate, and get 2% of people converting, whereas unlimited IAPs might only get 1% of people converting. But now some of that 1% can (and will, believe me) spend $30k a year on your game. And almost all of them will spend more than $10.
Games used to do this somewhat frequently, they just don't anymore because it leaves so much on the table.
Also the much lower RPUs will make paid acquisition unprofitable, and paid acquisition is the only reliable method. App stores are great if you get featured or by some fluke work your way to the top of the charts, but for 99.9% of developers, you're either doing paid acquisition or getting no substantial amount of customers.
Games used to do this somewhat frequently, they just don't anymore because it leaves so much on the table.
Also the much lower RPUs will make paid acquisition unprofitable, and paid acquisition is the only reliable method. App stores are great if you get featured or by some fluke work your way to the top of the charts, but for 99.9% of developers, you're either doing paid acquisition or getting no substantial amount of customers.