Is it easier to get large amounts of energy from fossil fuels than from other energy sources and why?

It's easier to get energy from things like gasoline because the fuel has a very high percentage of carbon. The other component of fuel needed for combustion is oxygen, but that does not have to be carried in a tank because it can be taken from the atmosphere through an air filter.

1 kg of fossil fuel burns with about 3.5 kg of oxygen to produce 3.1 kg of carbon dioxide and 1.4 kg of water vapour.