Well, it's not THAT terrible, not mine anyway. It's only about 2-3mpg worse than either the Honda Element AWD/5sp we had or our '07 Pilot 2WD with the cylinder deactivation, and just about the same as my wife's '13 Expedition.
Well, the Element is also sort of a box on wheels and the Pilot is a slightly larger and slightly heavier vehicle than the Wrangler.
The Element was significantly smaller than the Wrangler and not nearly as boxy, even though they worked really hard to make it look boxy. It's downright swoopy compared with a Wrangler.
The Wrangler is also bigger, at least in terms of wheelbase and overall dimensions, than the Pilot. A lot taller, somewhat wider, longer wheelbase if not longer overall. IDK the exact weight but my feeling driving them is that the Pilot is probably 400+ lb lighter than the Wrangler. There's a LOT of weight in those axles, the frame, and the wheels. The wheels and tires (counting all 5 incl. spare) alone are probably 400 lb heavier than the wheels and tires on the Pilot.
When I was driving the Wrangler I remember it getting around 16+/- with road tires. When I got the Cherokee, a... I would get around 20. ... It was also quite a bit lighter. I remember it being a much faster vehicle if you got on it, as well.
A 5sp Cherokee with that 4.0L engine definitely felt quick back in the day!
I think the 5sp had a lot to do with it, and being lower makes a big difference in overall wind profile.
Anyway, I get about 18 avg. in routine driving in my 4000-lb light-lift-and-33s 6sp Wrangler Unlimited. Not horrible. We got better gas mileage in the rental Wrangler Unlimited(s) we rented in Hawaii with hard tops, no lift, and much more highway tires.
The new Wrangler 3.6L V6 is a far better (more efficient, more powerful, etc.) engine than that old 4.0L I6.