If we carried only O2, what would we react it with to extract energy?
We have been burning hydrocarbons, (such as wood, peat, coal, fat or oil), in air for thousands of years. The air contains oxygen which is consumed by the reaction to produce H2O and CO2. So over time we have come to expect chemical energy to be released by oxidation of a fuel. The oxygen is not the fuel, it is the oxidiser of the fuel.
Hydrolysis of water generates a fuel, H2, and an oxidiser, O2. We can then recover some of the energy invested by reacting them together. But we do not need to carry the oxygen on our vehicle as there is more than sufficient in the atmosphere. Because O2 weighs many times that of H2, our total vehicle weight is reduced by carrying the fuel only, without the oxidiser.