Weight is important for two reasons (primarily): 1)It is directly proportional to rolling resistance. Even at that, rolling resistance is a small fraction of the losses due to wind resistance - and weight has nothing to do with air resistance. 2)Weight is directly proportional to inertia. Launching a vehicle takes a lot of engine power because of inertia. But once you get a vehicle to steady state, weight only affects braking and rolling resistance. So, a big car at steady highway speeds is capable of good mileage (my old Buick gets 30 mpg on the road at 70 mpg.)
Air resistance is proportional to the square of speed. The apparent frontal area of the vehicle X Cd is directly proportional to wind resistance. Most motorcycles have very high coefficient of drag while cars have been improving Cd for many years. Next is efficiency of motorcycle engines. Most folks don't really care as long as the bike is quick and fast. Bike engines are tuned for power not fuel efficiency. Also bike engines aren't held to the same emissions standards, (and no CAFE standards) and air cooled engines are inherently less efficient than water cooled.
Put it all together and there isn't much mystery though YMMV.
It's quite off-topic, but if y'all indulge me, I will explain why I still find the relatively poor fuel economy of a motorcycle unexpected.
First, it is true that weight or rather mass matters most during acceleration or going up a slope and does not matter when you maintain a constant speed. However, the rolling resistance of the tires increases with the weight bearing down on them.
The four big fat tires of a car with 3,000-4,000 lbs weighing down on them must have quite a bit more rolling resistance compared to the two motorcycle tires.
Secondly, a car body is more streamlined than a motorcycle along with its ride, but a car has a much larger frontal area. Consider the equation for the drag force.
Fd = 1/2 * rho * V^2 * Cd * A
rho: mass density of air
V: speed of vehicle relative to air
Cd: drag coefficient
A: frontal area
Cd for a modern car is about 0.25. For a motorcycle+rider, I saw a number at 1.8, which is 7 times higher than that of a car. But what's the ratio of the two frontal areas? Perhaps the motorcycle frontal area is higher than 1/7 that of a car, hence a motorcycle perhaps ends up having more drag.
Finally, let's look at the engine. I have not looked at the fuel consumption of a motorcycle vs. its output power, but let's compare a 400cc motorcycle to that of a car with a 2500cc (2.5L) engine. You can kick start a 400cc engine, but can you turn over a 2.5L engine that easily? The car engine has so much internal friction compared to the smaller motorcycle engine. I would imagine a car engine burns a lot more fuel just to turn itself at a couple of 1000 rpm, let alone moving the car.
Still, somehow the car does not do that badly, and that's what I found unexpected.