You're not exactly wrong, but you are kinda wrong if you judge based on the which bed length is most commonly purchased by consumers (as opposed to contractors). On almost any dealer lot, you'll find maybe 1 8' bed, and it'll probably be a base model 1/2 or 3/4 ton with a bench seat, perhaps 90% 5.5' bed trucks, mostly in luxury trim, and a small handful (if you're lucky) of 6/5' bed trucks.

The drivability of a 6.5 vs 5.5 is huge because of the turning radius resulting from the longer wheelbase. Most people don't want to have to think about where they can park.