Because it would be a disaster. There are so many games that you can complete in a day or two. They may be absolutely FANTASTIC, like Monument Valley. But people already don’t like paying for apps so they’ll ask for their $.99 back after they’ve played the game.
The end result is the only games they can survive or 90 our games that are full of in app purchases. Every other app could be easily download it, used, and refunded it quickly.
As bad as monetization is in the App Store right now it would be in order of magnitude worse.