The short answer is because then can. Most are run as a for-profit business and they do that very well. The same for girl’s seminaries, matza and esrogim. If people did not pay it they would charge less.
An additional factor here is that each camp feels pressure to out do each other in lavishness of the trips. Remember when a trip outside of camp was a hike or at most a bus trip to Woodburn for pizza (bought at the camper expense)? Now if a traveling camp does not fly to exotic places like Alaska it is a nebach. This does not answer everything but it all comes back to the level of materialism that WE create puts pressure on the weakest.
When it comes to camps and seminaries it is worse. You try explaining to your kid why Mom and Dad think it is insane to pay for a month of camp close to what they pay for a years worth of tuition. Let me know how it goes.