Like I said, it's because keepers are a different beast. If a young midfielder shows some talent for example, they will get a few minutes here and there in the first team and can easily step in and out the fringes of the squad.
Keepers generally are nowhere near their peak at 18 or 19, and nobody is going to give them 10 minutes of game time as a sub every couple of weeks. So it's a weird situation made even more difficult by the elimination of reserve teams. Now when you are too old for the U20s you really struggle to get game time.