The same reason why we let our children "skin their knees." If they don't break their skin they won't know why not to. That's a concept we can wrap our heads around so I just think of God as that on a much larger scale. I think all worthy sentient life will eventually be brought back after our species grows up enough to handle the population and all that comes from that.
Thats wrong IMO on several levels:
#1: If the parent was all-powerful, he or she would probably find a way in which their kid could learn to ride a bike, say, WITHOUT having to fall and risk breaking their arms. If I COULD prevent their pain as a parent, I WOULD. Its simply that my lack of power means they have to take some risks in order to progress.
#2: Your logic MAY be able to be twisted into something like: god allows us to get broken hearted during dating for us to find out for ourselves what love is. Or god allows us scientifically to search for answers to help our ability to learn and expand our capacities. There seems to be NO way to positively twist the reasons god allows hurricans tornadoes which rip people apart, volcanos to see them burn or suffocate to death, floods to make them drown, horrific, specifically designed viruses (say ebola) that cause people's bodies to erupt and liquify in a grotesque and brutally painful way. Or plague. Or leprosy. Etc. Equate that with "lets a kid skin their knee in the process of riding a bike"? Come on now.