re: Why would any parent push their child to play RB?

Yeah I don't think a parent is going to 1) put their foot down and insist their stud son play corner instead of QB or RB where they could help the team more (in the early years of developmental football especially, but even through HS); or 2) win that battle with the coach.

It seems like it's just a product of the relative development of different positions. In youth football, corners don't affect the game nearly as much as RB. QBs aren't developed so you aren't taking away as much value from the opposition. And there isn't as much physical parity so a RB can completely dominate a defense.

Now if they play the kid both ways, sure, best of both worlds. But at each stage prior to college (in which case they're auditioning for the NFL and get the most value out of exposure at their specific position), exposure in general is more attainable via the RB position than other positions that eventually become more valuable as the game develops.