Because there isn't that much else to do as a teen. The opportunity cost of spending time on gaming is low. You could get a job and start saving or investing, or study harder and advance your education more quickly, but it's not the expectation that teens will do this in a significant way.
An adult who is free to do anything and chooses to spend a lot of time gaming has a higher opportunity cost and a higher likelyhood of regret later.
An adult who is free to do anything and chooses to spend a lot of time gaming has a higher opportunity cost and a higher likelyhood of regret later.