Because it will always take more energy to break the water than you will get burning the Hydrogen in Oxygen back into water - it's basic thermodynamics.

You will lose some energy as heat that you cannot get back*.

You can't power a car from a process that loses energy. Even if you use a battery to donate the lost energy, then you might as well just cut out the lossy middleman and just run off the battery - which is what we currently do.

It is better to think of Hydrogen as an energy transporter than as a fuel, as you'd need to generate the Hydrogen somewhere that has abundant energy (ideally renewable), then transport I where needed, such as a Hydrogen powered generator.

*Interestingly the fact that all processes generate waste heat means the only theoretically 100% energy efficient process is heat generation itself, as all forms of energy eventually degrade to heat (as it is essentially the universe's waste energy).