Kids' ticket prices

WHY do kids' tickets at touristy place cost less??? They cry, fight, are slow, make a mess, cause more aggravation for the adults, and probably don't want to be there as much as adults. The really young ones won't even remember the experience. So, their tickets should cost MORE.

Is it because they take up less space? In that case, ticket prices should be based on size, not age. I'm probably about as big as an 8 year-old, but I complain less. So, I should pay half the price.