It's a really difficult subject to get into on here and explain, but development of goalkeepers is really difficult for a number of reasons, there's only 1 in a team for a start ( and you can't play out of position ) the #1 keeper in a team never really misses many games, keepers don't hit their prime until 30s so hard for young ones to break through past those older ones In their prime. Young keepers tend to have to drop down the leagues to get games/experience and move back up the leagues ( I know not always before anyone says )
The way goalkeepers are expected to play these days isn't the way most of them have been coached and developed over previous years in the UK so more keepers are coming in from abroad which again makes it harder for younger ones to develop and break through.
The numbers & % of young keepers who actually make it is miniscule