Why is a big unknown. But some of the answers aren't hard to figure out. When it comes to deviant sexual predators like the human pedophiles it's easy to take the next logical step and believe they would be more than receptive to a market in which their preferences (young kids) could stay that way. How many kids are thrown away when they reach puberty? How many die because they are no longer useful to a specific market? How many others resort to prostitution in order to survive one their pimps abandon them because they're too old?
In my shifter world, it's the same idea. There's a big market for adolescent shifters. It's not a big leap to think that there are predators (shifter and human alike) whose kink is hybrid adolescents.
Even though theoretically humans in general don't have knowledge of supernaturals, there will always be that segment of the population who are aware and will use that awareness to their advantage.
In our world there is a huge black market for children. Especially in other countries who don't have the resources to go after those responsible. It's the same in the supernatural world.
These are circumstances I wish didn't exist in either world, but it creates a realistic building block for my story. Some details will only be alluded to. I will never exploit young kids in detail in any of my stories. I just won't go there.
I'm sure there will be more information forthcoming that will answer a lot of they why's, but for now I hope this helps.