Do you know why public toilet doors don't touch the ground?

It stops people from doing things they aren't supposed to do. That includes getting frisky.

Since the stall isn't enclosed completely it discourages behaviour that's frowned upon in public. Like vandalism, graffiti, drug abuse, or even sex. Behaviour like this has actually decreased since such stalls started being globally used. People do realise it is a public place and refrain from doing something stupid in there.