Why aren't public restrooms fully enclosed?

Don't know why I thought of this recently, but isn't it weird that restroom stalls in America have a foot tall gap at the floor and ceiling? I mean, how many times has a single mom been caught with her pants down when their toddler decides to play hide and seek! Plus, I think that would help provide privacy when someone decides to answer the phone while we're peeing...

Also, if public bathrooms were fully enclosed locking stalls, would you feel comfy it being unisex? In my head, the only thing being shared in that case, are the sinks.

(I don't want to hear the argument about transgenders using whatever bathroom. You can never tell if the same gendered person in the stall beside you is homosexual or not. From my research, sexual preference and biological differences don't play a huge role in predatory behavior.)