Why cant they invent cars that run on water?

Water is inert and so cannot be burned or used in any other chemical reaction. There are people trying to mislead with the "so called" water power cars. When the water is split into its basic elements, those elements can provide power. The problem is that it takes as much power to do this as it gained from it and its not water power. Its hydrogen power. I think that there is merit in hydrogen power since mains electric is cheaper per kw than gasoline. Its simple to produce the hydrogen using electric and it burns with only water as a waste. A standard car engine requires only small modification to run on it. The only drawbacks are the storage issue and the public mis-conception that its more dangerous than other fuels.

Water is a byproduct/waste of combustion reaction. This is gross but think of consuming human waste to run yourself. You can't. Simple. Water is a substance which have a lower enthalpy than the reactants forming it, hence to reverse it you need to add energy rather than take out energy.

Because how do you extract energy from water, when it's already void of most everything (and has virtually no potential for chemical reactions)?

Water doesn't burn.

because it's impossible.

Cars have been invented that run on Hydrogen - when the Hydrogen is burned as fuel the "exhaust" is water. Water doesn't burn so cannot be used as a fuel

You could run a car on water IF you could produce a fusion reactor small enough. The fusion reactor would produce steam that would be used to make electricity that would run an electric engine and also separate the water molecule into hydrogen and oxygen. The hydrogen would be the fuel to the fusion reactor. If ( or when ) fusion power becomes a reality, it is more practical to produce electricity and run electric cars using batteries.

Umm actually, there is a car out there that runs on water. It was created by a normal person though. Not a government thing. Many people find there own ways to run their cars. Someone uses olive oil. They just aren't telling how they did it so it won't be used. If it is used then prices will get messed around with what they use and it would hurt their way of making a car run on what they use.

There is but the government cant make any money http://www.spiritofmaat.com/archive/feb2/carplans_doc.htm