Because you are ignoring basic science.
The internal combustion engine provides power by the expansion of the intake mixture to the exhaust mixture. This is accomplished by primarily 3 methods.
1) The breaking down of complex dense molecules into less dense molecules.
2) The vaporization of relatively dense liquid fuel into less dense vapor.
3) The expansion of gases due to heat: 
#1 is the same regardless of starting with a vapor or liquid droplets.
#2 is eliminated from from your claim, producing less work.
#3 is unchanged. It is still the same chemical reaction regardless whether you start from a vapor or a liquid. It may goes faster, but that does not produce more energy and the internal combustion engine works more efficiently with a steady fast burn over and explosion.
If you first heat the intake mixture, then the delta T of #3 has a lower ratio to the exhaust, providing less expansion and consequently less work.
Which is the reason Drag Racers used to cool their fuel by running the line through ice water prior the carburetor. Also, many of the drag racing records get set in winter because of the cooler temps of the intake air, resulting in a higher ratio of expansion.