Not to mention that if the keeper is using the smother technique (assuming they've started keeper training from about U9), they aren't going to really be solid on it 90% of the time until around U14....there's usually some error somewhere along the way. Until U12, the best they've gotten usually is maybe just an introduction to the technique (perhaps even only at keeper camp, assuming that's what they do)....before then there's just too much else to cover that's a higher priority including catching, diving, high balls, and distribution. It's just not in the rotation enough to master.
And even if the keeper coach is covering it, it's usually in keeper training without strikers present and let's face it most keepers aren't there because they are talented strikers...they are generally good all round athletes but if they have a strikers gift they usually get pulled to the striker role which is more valuable in the sport. So the training they generally do get is at half speed. My U12 son has had maybe 10 sessions to date at full speed, and that's not enough to master it. He has to learn the various different defensive techniques, the various different zones, and when to come out. Of the skills, this is one of the hardest things to master (which is why early on you see so many of them either frozen to their line or coming out feet first). What's worse is that once they have it, the strikers are also learning other techniques to counter (such as the toe pick or the lofted chip) which have to be added into the equation (so my son has had to relearn the technique with every new adjustment he sees because he has to add consideration of that new technique into his judgement).
Then you have the players that once the keeper has executed a dive and is the ground, will try to kick it out of his hands. The issue with the GK is he's on the ground which makes him vulnerable to a kick as well as makes it harder for the ref to see it. It's not that the ref needs to give the GK special protection because he's the keeper, but I would hope that the ref would make an effort to discourage an dangerous play against any player whose prone (it's just the GK is prone more often than other players).