The side effects and risks are worth it when you only get one shot at puberty. If you don’t transition as a pre-pubescent teenager, you will never “pass” as well, especially as a transwoman.
A really good example of how successful you can be if you’re early is Corey Maison. There are cases of transition regret of course, but they’re still a very small percentage of the total, and that percentage is reduced by puberty blockers giving young people more time to figure out themselves and their own bodies, and to make the choice that’s right for them.