Reasons I can think:
* Harm to the body
* Harm to the brain
* It can be expensive (sometimes disastrously)
* Opportunity costs, both of time and money
* Drugs alter your behavior, sometimes not in polite ways
* Addiction make you dependent on something outside of yourself, a literal substance.
* Many drugs leave you unable to work or operate heavy machinery (cars)
* Some addicts prioritize their drug over anything, even family or friends.
* Some addicts with steal to fund their habit.
Some people make it work, there are high functioning stoners out there, there are people who need painkillers for chronic pain. These people don’t let a drug affect their lives as negatively as the alternative.
But being a dependent addict isn’t virtuous.