thats a very interesting point you have raised buddy. and please dont pay any attention to the ones who think this is stupid. it is infact a very good question.
Kindly allow me to offer my two cents on the subject:
generally car engines are more efficient than bike engines. car technology is older than bike technology (commercial sale) and hence car tech has evolved much more than bike tech.
secondly, bikes are usually categorized in two different slots. either they are economy drive models which are sold so cheap/inexpensive that it is pointless for manufacturers to do to research for better and more fuel efficient engines OR they are made so expensive that the manufacturer is only interested in higher RPM/Torgue/HP/performance and is least interested in fuel efficiency.
then comes the actual tech differences:
if you look at our single cylinder thumper , you find that the engines, fuel delivery systems and similarly drive terrains are utilizing technologies that were first used in in cars in early 50s and 60s.
for example: electronic fuel injection systems, computer controlled ignition timing, variable valve timings and multiple valves for intake and exhaust. these systems are almost never available in our locally produced bikes.
now just imagine a commuter bike with following features and imagine what the fuel average would be like:
CCU electronic fuel injected 250cc 4-valve single cylinder bike with computer controlled dual timed ignition (two spark plugs) and variable valve timing. now just imagine what the fuel average would be like in such a bike. this particular bike (hypothetical) calculates and adjusts its ignition timing, air-fuel mixture ratio, different valve timing for different RPM levels, analyzes exhaust gases and spark is produced through 2 plugs instead of one to promote a better burn. I am sure you must be thinking pretty impressive mileage right?
well...thats only one part of the equation, are you considering the actual size of the head to accommodate 4-valves? have you considered the overall volume of the engine? can you accommodate such a large engine on a bike frame such as the one on a 70cc or 150cc even?
are you considering the cost implications? would you pay something like 200k for a bikes that does 100km/liter? how many of us can spare 200K or even 150K for a bike?
any industry (commercial) usually runs on the basis of "money makes the mare go". if the industry thinks that they can make money by introducing such a bike vs the cost to make such a bike (bike's cost plus research and development) they would happily jump right on it.
unfortunately, the dynamics of our market are as such that you get guys who want a daily use bike to go to office or you get guys that are insisting on converting they 70cc bikes into crotch rockets by taking everything off and putting in bigger engine heads and 125 carbs. OR you get guys that are more than happy in converting their 125cc into super bikes through over boring, bigger sprockets and free flow exhausts.
now can you convince these guys into buying a bike with super economy but no initial pick or acceleration? can you convince such guys who have removed their bike's silencers just so that they can enjoy the roar of the engine? lols!