Why do people have lawns?

I wonder why people spend so much time making their lawns green instead of making them into a garden or something like that. Also, why does it have to be grass? A rock garden would be far more pleasant, in my opinion.

Most lawns are too small to be used like a lawn, e.g.: for playing with dogs, throwing a frisbee, or lounging with a group of people in the sun. Wouldn’t it make more sense to have a few households own a lawn in common, so it could be larger, and better maintained? It could even become something people worked on together to beautify their common living space. It would probably also make lots cheaper, and houses in that neighborhood more valuable.

It seems like people have lawns mostly because the houses they buy conventionally have lawns attached to them, and they then feel obligated to keep their lawn somewhat healthy. Is this the only reason? And even if it is, why have “lawns” become conventional, when they seem so silly and wastefull?