Most of them aspire to be in the NFL or NBA but in between the two (mma and boxing), I think youth still want to be boxers more.
MMA training is expensive. It requires training in several diciplines and thats why most MMA gyms are in the suburbs. Suburban kids usually have more opportunities other than fighting for a living so MMA isn't really like a job for them.
Boxing, on the other hand, is inexpensive. Most gyms I know charge very little for training and, in some gyms, they'll train you for free if you do some work around the gym.