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H2O is a molecule. It has covalent bonds/hydrogen bonds. Yes it naturally does form loose H+ and OH- ions suspended in itself (why water conducts) in a equilibrium reverse reaction but...
Firstly, the resistance of pure water due to its very low ion concentration is extreemly high and secondly the ammount of current consumed to produce the gas is also high. Not to mention that the reverable reaction is extreemly slow meaning that hydrogen is produced slowly. Do not forget to factor in that the conversion is less than 80% efficent. Electricity is also extreemly expensive, unlike oil.
So to cut costs, industry devised a very smart process called steam reforming. In this process, they get a long hydrocarbon chain and put it in the presence of a steam (as well as a catalist to lower activation point) to release hydrogen and harmless carbon dioxide. Usually this is done to methane (yes the same gas your digestion produces) from natural gas. The result is cheap hydrogen at upto 80% efficency.
Ok so both these methods seem equally efficent? No they are not as electrolysing water uses electricity from powerplants (often fossile fuled) with atmost 70% efficency (gas can obtain that). Thus there is a double wastage with the process and thus why it is uneconomical. Additionally if you want to use electrolysing water to fuel a hydrogen economy, you will have to build more power stations, like nuclear to keep up with the energy demands.
Currently research is being done if it would be possible to recycle carbon dioxide to a degree by using it to stimulate photosynthisis. A major limiting factor in photosynthisis is the concentration of carbon dioxide (it runs so fast, the stuff can not diffuse fast enough), thus by pumping exaust fumes from processes like steam reforming and fossil fuel power plants into sealed off areas with plant mater (like alge vats of greenhouses), you can actually increase their yield and thus boost the efficency of the process (as you can then factor in the energy it is permiting to be trapped via photosynthisis that is normally lost). You can then harvest these plants and convert them into bio fuels to help power your cars.
Additionally, an emphisis on car efficency will eventually occur. Often on the road you are stuck most of the time at sub 100 km/hour speeds so why make cars that can go faster (although efficency and speed are linked to a degree) if there is efficency to be gained (there might not but who knows).