I don't completely disagree with you by any means, but there are lots of things we don't leave up to the parents because all of them can't be counted on to make good decisions that are in the long-term best interests of the child. We don't let parents decide when their minor child can drive, drink, smoke, and in cases of just choice, not for direct medical reasons, be sterilized. A child's views and wants and ideas of what they are or want to be will typically change dramatically over the years, and to make major irreparable changes to their body via surgery or puberty blockers before being old enough to fully understand the consequences is dangerous and to be avoided even if the parents are OK with it (or sadly, pushing it for their own reasons).
We don't let kids make major decisions in most cases because they don't have the experience, wisdom, or grasp of consequences to do so with anything approaching "good choices" so to speak. They're also VERY susceptible to manipulation by adults, especially parents. Parents deciding to allow their minor child to have trans surgeries or take puberty blockers for non-medical reasons is VERY prone to being more about the parents' needs than the child's, and thus the government's role in prohibiting it. I'm generally for smaller government and them being less intrusive into our private life, but I'm also for them protecting children against bad parents too. I just don't believe any kid should be doing something this permanent before they're old enough to fully understand the consequences, even if the parents are OK with it.