But what is their reasoning for this?
For most areas, it's aesthetic.
Here in the south-east, a short lawn will have no mosquitoes but a grassy area will have plenty (as in you'll get 20-30 bites in one evening outdoors), so that's a strong motivation.
Simply a case of no, a lawn it must be?
... which means "protect interest of neighbors".
People like when their neighbourhood looks nice and is safe from snakes, bugs and mosquitos.