The fact that goalkeepers need specific training doesn't make their skills harder.
How does that follow? I think most people would agree that the harder something is, the more detailed instruction you will require to do it well.
Think Rabbit gave up, so I will answer.
Goalkeeping does require separate training but that does not make it harder. It's simply at junior level everything is pointed toward the outfield for natural numerical reasons. It's unlikely that a kid will come across specialist coaching until he becomes part of an academy.
Get a maths professor to teach kids Maths & English and the kids are more likely to excel in Maths.