I know this one!
Lawns used to be a status symbol! Beginning as far back as the 16th century in France and England, the very wealthy would show off how wealthy they were by maintaining huge areas of their land covered by useless grass that wasn't being used for grazing, nor were the patches covered with lawn being used for food production or other such 'normal person' concerns. And it cost a lot of time, money, and manpower to maintain those useless stretches of grass, so that was another way to flaunt wealth.
And of course everyone started trying to emulate the rich folks...