The answer to your childish “Why?” question lies in the answer to my question, “Do you drive stoned?”
And if, as you claim, you don’t use drugs then what is your problem?
So you think legalizing will lead to more stoned driving? That assumes that people who didn't smoke when it was illegal will go on to drive stoned even though that's illegal - which is illogical and one might even say "childish."
And if, as you claim, you don’t use drugs then what is your problem?
Already explained: "I can tell you the harm of keeping drugs illegal: it hyperinflates drug profits and channels those profits into criminal hands. (Secondarily but not negligibly, it incentives the the creation of new, poorly-understood legal-for-now drugs and their use by those seeking mind alteration.)"