Motorcycles can be insanely efficient, some pretty ordinary ones achieve well over 100mpg.

But internal combustion engines themselves are pretty maxed out on efficiency, in the mid 30% range.

At least in the US, motorcycles are more often toys than tools so people buy bikes that emphasize other characteristics at the great expense of efficiency, and the more practical and efficient options are not even for sale.

Bicycles have 200-300W to work with. Motorcycles have 50,000-125,000W to work with. One puts a far higher premium on efficiency than the other.

If you optimized a motorcycle for a top speed of around 30mph it would be very efficient too (and look a lot like an electric bicycle).

Friction, weight, engine, aerodynamics