You do realise that it's possible to believe both "the world would be better if everybody did X" and also believe that "the government should not have the power to mandate everybody does X"?
I can imagine the argument when X is something that only negative affects the 'defector' (eg: "everyone should wear a helmet but the government shouldnt make you wear a motorcycle helmet") but what's the rationale when the reason that everyone should do X the social good at no costs, rather than personal good?
ensuring compliance has a non-zero cost. you need to set up a regulations, have people to enforce them and deal with the aftermath of the enforcement (this last one is why the war on drugs was so dumb).
Because all government power is itself intrinsically a huge cost to society. We don't serve them, they serve us.