Whereto start?
1. Make it simple then. You can't have an elite keeper because at age 8 and 9 all keepers are beginners and they should be part timers. That's not true of the field players since some kids have been training since age 3 or 4.
2. Are there kids that are better? Sure, but the criteria at that age for goalkeepers is different. Why? Because the more advanced kid at that age will actually be giving up more goals. We are sort of constrained here by the parameters originally set by Emma which was there is a distinction between the "superior" players playing on higher level teams than on the lower level teams. Espola's point that there could be superior players playing on lower level teams is actually equally valid, but that's not where this long winded side debate began with Emma's post.
3. Soccer development is not linear. Much like physical growth in kids, it comes it spurts and stops. One of the biggest spurts for the goalkeepers comes with puberty. For the boys, being able to touch the cross bar is actually very useful in ability to stop goals.
4. There isn't a shortage of keepers on the boys side...that's mostly true of the non-elite levels for girls. There are a shortage of trained keepers on the boys side but there are keepers a plenty.
5. The amount of stuff a GK needs to learn just simply exceeds that of a field player. They need to learn soccer + the goalkeeping skills. So yes, it is very hard to be an elite GK at the highest levels. They also have a shorter time period to learn it all, particularly if they start late, which makes it all the more impressive.
6. The salary stuff is an irrelevancy. You assume that soccer is actually a meritocracy (it isn't). If you read soccernomics, you'll see that the goalkeeper actually ads only mediocre value to a team because they can only stop (some) of the shots and so are only reducing a hole the team is in. To win, the team needs to score. That's why those up front pull in the highest salary. It's basic market economics.
Typical field player parent.
1, 2 -- not true.
3, 6 -- irrelevant.
4, 5 -- what's your point?