I am not a professional, just a rank amateur. But, I am never at a loss for opinions. I would speculate a few reasons and zhounder gave you a very good. Another is competition; Once an assistant learns your client base and how you make your profits, then they are a danger for competition. Probably not a good reason but a reason. Another is the talent .vs. skill issue. A young kid with talent and imagination can be a threat to any photographer. It ain't that hard to take A good picture, but it can be darn hard to take THE good picture. That skill does not come easy and a Pro will be reluctant to hand it over to just anyone. Can you blame them, that gives them, "the edge." I'd say keep trying, there are plenty of photographers out there and at least some of them should be willing to give a kid a chance as long as they promise to not be too big of a pain in the ass and are willing to do the grunt work in exchange for minimal pay and valuable education. Good luck to you and remember this question when you are asked it 20 years from now.