They just lag behind the skill level of the regular players, or rather, it's not actually their skill, it's their height.
There are keepers on good teams that are 1.7m tall. The England national keeper is this height. My sister towers above the national keeper for England. Goals are scored through no fault of their own, except for not being taller. Some of the "worldy" goals of every tournament aren't just savable, they are savable by female keepers of similar ability to the ones playing, if they were just of above average female height. It is astounding to me that national team keepers aren't even above average height for a woman.
Since keepers are drawn from a pool of outfield players after demonstrating some ability when younger, there needs to be an even more competitive pool of players such that that liminal case of players who are tall, agile and coordinated actually produce a few who want to be a keeper. Compounding it is the fact that that set of skills and height sets you up with more of an advantage in other sports, excepting specialized positions in football. So, volleyball, basketball, netball, even swimming etc. etc. etc. At a fairly young age, the tall girls get snapped up by interest in other sports.
So, the general state of play will have to be quite good, or the development of keepers on a national level very very prescriptive, to make a good tall female keeper. It'll take a while, and then a little longer still to trickle down to the worse teams.
