Let's pick drugs. Half of the drugs regulations deal with keeping you from killing yourself, which your approach would simplify (although sounds like a variant of "let's solve stupidity by removing warning labels and let the problem sort itself"). The other half, however, is how this change would affect society as a whole, and drug addicts are a burden on society (see: homelessness, hospital costs, correlation with violent crimes, impaired drivers hurting others, etc).
Of course, we know you would never become a dangerous addict lying in the streets arguing with yourself, but you can be damn right that I almost certainly would. And that's why, as I see it, Society (represented by the Government) jumps in and says "no drugs for either of you".
The other points follow a similar line of thinking.