Because some of us got through our teenage lives, and our adult lives, without needing to indulge, or experiment, with it. Anyone who thinks that marajuana is a harmless drug, is a fool. It's a drug, ergo by definition, it is harmful in some form, just as prescription drugs are and any other drug you can name is. I think the same about alcohol and have been a teetotaller for almost all my adult life. I think there is no difference between either drug, they are both harmful. People who indulge in one drug and carry on about how "this drug is not as harmful as that drug" are deluding themselves.
The bottom line is that I have lived, and continue to live, my life without drugs because I have never needed any of them. I depend on ME, not some drug to relax, or to temporarily change my personality, or to cope with life. Anyone who experiments or indulges in any drug, has a need to do so. I am not talking about an addiction, I am talking about a need to either do what everyone else is doing, or a need to escape, or a need to find a way to relax, or any other number of needs one can think of. I know that as someone who has never tried marajuana and doesn't drink alcohol, I will be ignored or ridiculed by those who do indulge, for whatever reason, and that's ok, I don't mind. I have political discussions and I am howled down on a daily basis in my staff room....I cope

(I am the oldest person in my faculty, surrounded by the youngsters who think they know everything, lol. They all drink....the women go home to glasses of red or white, the men go to the pub and, as I don't drink alcohol or any other drugs, and as I am pro-Israel and don't understand parents who nurture their teenagers "anxieties" and make excuses for their teenagers who suffer from LSD (lack of self-discipline), I am at loggerheads with my fellow teachers on a daily basis. I could never be a Year Advisor....my pastoral care would consist of "Are you over it yet?"
Anyway, perhaps I am not the person to be making a comment on drug use, having never indulged, but, just can't help myself.
My son-in-law is even more conservative than I am....probably why we get on so well and I really like him. His only comment on drug use is that firstly, drug users no matter what the drug of choice is, are losers full stop. If a person wants to take drugs who cares, however, if they OD or need help it should be denied. No ambulance for overdoses, no health treatment....made you bed, lie in it and suffer the consequences of your choices. He was in the Army for 10 years (he met my daughter when she was in the Army) and he is a teetotaller, never drank in his life, never smoked and never indulged in other drugs. No wonder I like him so much! Unfortunately, both my son and daughter occassionally drink alcohol...sigh, I went wrong somewhere!