The efficiency of fossil fuel energy sources is in their energy by volume. That’s is and has always been their main benefit.

While it is true, that their energy density is high, often it is actually much less relevant due to the poor efficiency of the devices burning said fuel. With cars, you’ll notice that ICE cars have a pretty terrible efficieny themselves, which partly negates the effect of high energy density of fossil fuels. These days electric cars and ICE cars can have very similar driving ranges despite their differences in energy density.