From Gordon Gray and Popular Mechanics: “Dr. Simpson explained the lack of Category 6 (or 7!) storms in a 2001 interview with the Sun-Sentinel by putting it this way:
I think it’s immaterial. Because when you get up into winds in excess of 155 miles per hour you have enough damage if that extreme wind sustains itself for as much as six seconds on a building it’s going to cause rupturing damages that are serious no matter how well it’s engineered. It may only blow the windows out, but on the other hand, it can actually rupture the stairwells, the elevator wells and twist them, and it’s happened in many buildings so that you can’t even use the elevators after they’ve experienced this. So I think that it’s immaterial what will happen with winds stronger than 156 miles per hour. That’s the reason why we didn’t try to go any higher than that anyway.”
Insurers already know from their own records that a 20 percent increase in peak wind speeds from 50 to 60 mph causes not a 20 percent increase in windstorm claims but a 500 percent increase instead.
A lot of that is knock-on damage. Debris blown loose from one roof at 50 mph may fall to the ground and slow down before hitting another building. At 60mph, it may hit another building before falling to the ground. And so debris damage can cascade.