Their body, their choice. Why should people who can use it responsibly be denied? And worse, why should they be punished if they choose to use it regardless of legality?
Because we don't live in isolated universes of individuality. When they cause massive damage to their bodies and minds, that cost is not only borne by themselves but by their close circle and society at large. You can argue that prohibition is not the right approach to preventing these problems, but doing the non-optimal thing is better than doing nothing at all.
e.g.
Alcohol is worse than no alcohol, for many reasons.
Smoking cigarettes, obvs.
Eating meat is worse that not eating meat.
Doing woodwork without breathing protection may damage your lungs.
Cleaning with spray cleaning products regularly may damage your lungs.
Sun exposure increases your skin cancer risk.
Running is more likely to cause arthritis than cycling or swimming.
Loud environments may damage your hearing.
Insufficiently frequent ejaculation may increase your risk of prostate cancer.
Yes, I'm being deliberately absurd to make the point, but still: where, and how, to draw the line?
Libertarianism as an ideology does not have the tools to deal with the harms of the latter.