Why are motorcycles street legal and not ATVs?

At least where I live, motorcycles are obviously legal, and ATVs aren't. Ive heard the argument that people are recless on them, and i think it's due to the fact they are illegal. If you put signals on them and whatever else needed to be street legal they should be, they have 4 wheels instead of 2 and they are pretty bulky. They seem overall about as safe if not safer than motorcycles.