No, no specific reason, probably just convenience. I always bought plant stands to add height to my plants and keep them off the flood, which made cleaning the floor sooo much easier.
But you can hang them from the ceiling, definitely. You'd have to suspend them from something like a wooden dowel that crosses the whole top (resting on the frame at both ends) so it won't collapse your screen/mesh. But it can certainly be done. People use something like strong fishing like (like the 100lb kind) and just pass the line through the mesh without ruining it.