There are basically two obvious possible explanations for why kids don't get off puberty blockers once put on. One possible explanation is that the screening process works, so only kids for whom the treatment is appropriate go on them in the first place. The other is that being on puberty blockers locks the kids into the transition path. Given that the screening process was not refined over time to reach that virtually 100% state, I find the latter explanation much more likely than the former.
Now, as to the detransition rate for such kids, that's a harder question, because nobody has really looked at it. If you go on puberty blockers, transition to cross-sex hormones, and then detransition later, many of those studies I mentioned wouldn't pick you up. And there's been almost no long-term studies of what happens to these kids. Furthermore, the detransition rate for kids who have gone through with treatment may be different than what the detransition rate for those same kids would have been had they not gone through treatment. The treatments cause irreversible changes. I have to wonder how many regret transition but don't detransition because they can't fully go back to their natal sex. Or even if they don't regret it, how many would have still accepted their natal sex if not pushed through the medical path early.