It's true people need purpose in life. They can find purpose in art, in travel, in maintaining a personal garden, in maintaining connection to their community. Not through bullshit jobs. What's the point of humanity if we can't truly automate the toils of physical and knowledge labor to somewhere south of 10 hours per week? What's all of this for, anyway?
Let people have their legal weed and live a little. We don't need to work so hard. We've achieved so much innovation in the past few thousand years, we ought to enjoy it before perishing, or continuing focusing our lives wholly on work.
If you really want to think of it as a system, then the proper way to think of drug usage is as an escape valve for societal issues like loneliness, hopelessness, etc. If so, then it's important to legalize it so that it can be monitored and measured; if it's measured, then society's government can focus on reducing that measurement as low as possible by treating the systemic issues and not the symptom.
Look around yourself, how many people are fat? That is how well people will responsibly use drugs. A large majority lacks the self control to properly use these substances and the law helps them do it.
Your comparison to the obesity epidemic is a non-sequitur. People abuse drugs to fill an emotional hole in their lives. While that does sometimes happen with food, most overweight people in the US are generally happy. The obesity epidemic isn't caused by people abusing sugar to escape unfulfilling lives; it's caused by callous government policy that makes healthy food expensive and junk food cheap. Too many Americans genuinely lack access to healthy options, let alone ones that are economical.
Everything hurts others. People and their actions don't exist in a vacuum - we are all players on a team called "society"; the actions of people, even if done privately, can end up affecting the performance of the team. Cars, alcohol, and tobacco are all legal, and all of those things end up harming others directly or through second/third order effects.
I'm not necessarily against decriminalizing or even legalizing marijuana, but I don't think the reason for doing so should be because it "doesn't hurt others", but rather because governments should infringe on liberties as little as possible even if it means bad consequences could emerge from abuse of said liberties.