Perhaps the most brief answer is that you're adding mass (a generator and electric motor) with no increase in power. No energy exchange occurs without energy loss, either, so you now need a larger engine to achieve the same output power to the prop.
I drive a plug-in hybrid, and it's very good at my "mission". I can round trip to work without gas, yet if I need to go see the kids at college I can make that round trip with a combination of battery power and a bit of gas. The engine runs more often than not on longer highway trips, but the battery still takes the edge off going up hills or passing, while the engine cuts out and the battery recharges down hills or decelerating. Bottom line, a highly variable load (speeds, hills) suits the hybrid power train well.
If I just set out on the interstate doing 75 mph for hours across states like Illinois and Indiana (flat land), I get worse mileage than a car running purely on gasoline. That's because I'm hauling around ~500 pounds of electric motor and battery, but I *need* a nearly constant power output. This is analogous to the air plane example. With today's tech, you just can't match the energy density of hydrocarbons with batteries.