I suspect one of the main reasons is that you would get charged if you left your screen and left it running by accident. Imagine you were playing your phone game while waiting for the microwave to finish. You hear a pop, quickly put down your phone to make sure your oatmeal didn't explode. You see that your oatmeal is done, so you sit down on the couch and eat it, getting sucked into a good show. 2 hours later, you pick up your phone, only to realize that all your game time is done.
This would not be a good situation as a customer. With a monthly subscription, you are putting an upper cap on how much money you'll be paying, and you don't have to closely monitor your playtime. Customers who aren't closely monitoring their playtime tend to get more involved in the game, and will likely spend much more money on other game-related purchases like DLC or items.
Furthermore, paying for playtime makes you feel like each second spent in game needs to be worthwhile, so small annoyances or farm-ey gameplay would seem extra tedious, and be actually costing you directly. It would make this downtime seem absolutely unbearable.
For games with any form of strategy, one of the joys (at least for me) is to stare at the screen in-between action and plot how to maximize my power, or plan my next acquisitions or upgrades. If I was paying per minute, this would probably never happen.
For multiplayer games, waiting for other players, although already very annoying, would be absolutely unbearable if you were paying for that time.
