The reason is that they set a limit to current ability and the number of high potential ability due to league reputation and strength. It's stupid and underrates so many players.
But league reputation has absolutely no link to the numbers of prospects said league produces.
Brazil isn't a great league and yet nobody would dispute it procudes tons of talent.
It also depends a lot which countries, national teams, competitions and leagues you have selected to be shown in full detail.
I always choose at least Brazil, Argentina leagues, Copa Lib, African & Asian CL and all teams in continental competitions plus about 100 National teams fully loaded in detail. So I have about 500 000 players in my save.
Thus I see wonderkids from lots of places in my saves.
I do too, and England still produces the most wonderkids. Not forgetting that you often get insane players out of Ireland or Scotland because...
You must have a super computer.
Here's a not so well known hint :
The quickness of FM doesn't depend on the number of players, but rather the number of simulated leagues.
When you start a new game, choose "advanced" when you get the size of the DB choice. There you can choose for instance "every player from South America". Then you don't have to use brazilian or argentinian leagues as playable.
It will write that the CPU will be slower, but in fact, it will be quicker.