In many techniques in Aikido (and, IME, a bit moreso in Ueshiba's Aikido), sometimes a high breakfall is actually easier on the uke than a realistic response. A couple of our throws, for instance, have a couple of probable, realistic outcomes: crumpling forward awkwardly to avoid the lock/pain, or a break/dislocation/tear. A high breakfall avoids all of that. It also happens to look cool, and feels amazing when you're doing the throw. Those last two points are bonus side-effects, but the high ukemi does actually (sometimes) serve a specific purpose for the uke.
That said, of course, in a demo the breakfalls are even higher. I think it's the extra gravitational pull created by the audience. I'm pretty sure of that, because the height of the falls often has a positive correlation with the size of the audience.