The construction is different, shorts need to have the elastication around the top to keep them in place around your waist and on your hips, this has an obvious effect on the styling. This isn't totally similar to regular shorts, given that cycling shorts need to be designed for a body that's probably bending over a little more than normal so the top of the shorts at the back will be higher (hence also that cycle shirts have a longer back to help cover the inevitable gap that appears when you bend over).
So the bibs help with not needing loads of elastic that digs into your sides because they're being held up from your shoulders, but also the construction covers your lower back (including added insulation in winter/cold-weather varietals).
Not only are the shorts being held from above, they're being anchored in place so they won't rotate and ride around your sides - which is even more annoying than tight elastic.
It's similar to comparing an open-top car against its roofed equivalent - all that extra strength that comes from the roof has to be pushed into the superstructure around the new top of the vehicle. So it is with shorts.
Yes, the call of nature is trickier, but I'd rather be comfortable for hours and uncomfortable for a few minutes, than put up with shorts.