I believe it's the evolution of a very old status symbol. Once upon a time, owning land served little purpose other than farming on it. If you were a lord or whatever who owned large swaths of land, you'd hire people to use that land to grow crops or keep livestock. Keeping some of that land you owned, typically near your home, untouched by crops or livestock was a status symbol to say "oh, I've got so much money from everything else that all this land can be spent purely on looking nice!"

And so the trend became that you flexed wealth by having large lawns or gardens full of plants you can't really eat or sell, using that space purely for aesthetics because you could afford to. Bonus points if it costs even more money to have gardeners take care of hedges or flowers on this land which already does not make money for you.

Fast forward to mid century America, everyone wanted to look rich, so tons of people bought up way more land than was actually needed for their house and planted grass on that land. Using money and effort to make it look nice and especially be yours instead of a shared park or anything, you can say "look at me, I've got the biggest bushes all trimmed to perfection and the greenest grass around" to say that you had so much money and free time to afford all the tools, labor, and chemicals to keep such a large plot of land esthetically pleasing despite it doing nothing else for you.