2nd hand smoking is still dangerous, and you can still get high off second hand smoke around weed, if it's legal then parents could still smoke around children accidently getting them stoned
even once its legal, it's still illegal for "young adults" because you have to be 21 years of age to get it, and if they get it from someone else that's just as illegal, if not more illegal than giving alcohol to a minor.
although it is rare to become addicted to marijuana, it can still happen, and as your tolerance goes up you don't get the same high, that's why it's considered to be a "gateway drug" and can lead to more serious drugs that can cause serious problems
The legalization of marijuana would create an insane amount of income for the government. the amount of tax we would get would help us get out of debt, feed the hungry, and build the wall.