[athenot]

In some areas, tall grass becomes a habitat for snakes, which can be problematic for children.

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.

[pvaldes]

On the other hand, snakes eating all the rats would be good for children. Rats are reservoirs for many human diseases, but the think on the snakes!! is a common alibi for forcing your neighborg not to having shrubs, roses, grass, climbers, rocks, ponds and everything that would make your own garden look duller by comparison.