The Beaufort scale is designed to grow as B^1.5, and so it has a natural direct extension. 13-16 on the extended Beaufort scale do not map onto category 2-5 (https://en.wikipedia.org/wiki/Beaufort_scale#Extended_scale). As others mention, SSHWS was designed to reflect building damage.
Apparently, Saffir's original scale only included wind speed; Simpson augmented it to include storm surge and flooding. However, the categories were at times conflicting (e.g. Hurricane Charley in 2004 was a category 4 that led to a storm surge of 7 feet, or Katrina that made landfall as a category 3 but led to a storm surge of 20+ feet in places). To reduce public confusion, the NHC simplified the scale to only use a single prescriptive factor, wind speed (oddly, while still keeping Simpson's name attached to the thing even though they removed his contribution).
