Because it is a bit pricey! Don't forget adding an ECU, pressurized fuel system, etc. And I can go as lean as I want via cheap lil ol skool jets.
One reason fe is poor on my Wing is a suspected rich-running carb. They're kinda a bugger to remove and service, and parts are rape-me expensive.
Remember, Honda hates their customers.
Besides the shaft drive being less efficient than chain or belt and gearing not as tall as it could be, I don't think there's a fundamental (architectural) reason for the 1100 to be so inefficient. The carburetion is poor on mine but the ignition and exhaust systems should be good for effient fe. I think it's probable that a cam profile more biased to lower rpm operations would help along with intake system resizing (smaller, longer tubes).