I see it as each of the challenges requiring someone with the correct skills to be able to pass, so they need to know how not to panic under stress, how to solve logic problems, to be able to put themselves in vulnerable positions if it will help others, and to be curious about something for curiosity's sake.

Someone who knows the philosopher's stone is at the end of the challenges is unlikely to meet the last criterion, and usually, no single person could manage all the rest of them themselves. So long as a group of curious children didn't try to solve them together, things would be very well protected.