Aesthetically, you may be right. But yards are owned by and large by people with families. They have an important function to fulfill which is ruined by filling the space with plants.
However I rarely see anybody actually on those beautiful, irrigated lawns in Southern CA. People are usually inside or in an area with tiles or concrete on the ground.
There was grass in the mowed area, but also many, many other plants and insects. It was wonderful, as was the forest.
I find pure grass lawns very bland and unnatural, and in no way superior to a trimmed lawn with whatever plants happen to be growing. And you can have wilder areas too.
Perhaps you didn't mean 100% grass lawns, but that seems to be what a lot of American lawns aim for.