Why do all shower cubicles have open tops, and does it have to be this way?
Showers don't have to be open. I've seen and worked on many that had walls to the ceiling. 
I had a customer who had an open shower and inquired about closing it in because they had a breathing problem and wanted to breathe the steam. I enclosed it by adding two pieces of plexiglass mounted on top of the rails for the shower door and wall up to the ceiling.