I feel left out. I don't notice how fast the engine is running at highway speeds on my Helix. I've done multiple one-day 1000+ mile trips on it comfortably and was incredibly sore on a 750cc cruiser within 50 miles.
In other words, some people find the engine RPMs to be much less of a problem, if at all, than other (dis)comforts. For quite a few people, riding is all about some image that includes low RPM noise. As it was mentioned already, maybe some bikes are more vibration prone than others at certain RPMs due to resonance and damping characteristics?