I think it's about trust. Who the hell is stupid enough to trust any government? The government is made of men, and "If men were angels, no government would be necessary".
I do believe that adults should have their lives in their own hands; the relevant difference is that seatbelts, helmets, and fire codes do not go inside of a person, nor violate the sovereignty every person has over their own bodies.
If the government can compel by force (as all power and authority ultimately boils down to the force a person or entity can muster) that they are in charge of my children's bodies, what's to stop them from taking my children's bodies? For experiments, for war, or to satisfy the perversions of whoever happens to be in charge?
There is a point where the government or community needs to step in to ensure the wellbeing of the weak and helpless. That point is when it's clear beyond a shadow of a doubt that their current guardians do not have their best interests in mind, and not a moment sooner. And especially that should be the most drastic measure held under the utmost scrutiny.
If the government can compel by force (as all power and authority ultimately boils down to the force a person or entity can muster) that they are in charge of my children's bodies, what's to stop them from taking my children's bodies? For experiments, for war, or to satisfy the perversions of whoever happens to be in charge?