Why are new forms of fuel and power almost invariably associated with cars?

Fuel cells have been already implemented on submarines, military vehicles and busses, whereas automotive applications are still on a prototype stage. Also there are probably more electric motorcycles than there are electric cars. So I'd say that your blanket characterisation is not entirely true. Still, there is merit to the point you're making. The most obvious answer is the sheer size of the automotive industry and the amount of R&D funds it commands. So, even while cars are not the pilot application in many cases, they are the target market and design choices are made with that in mind. Also, while I don't have any figures available, I believe that most of the oil we are consuming nowadays is going to cars and to a lesser degree, to planes and ships. For cruising autonomy and safety reasons, planes and ships would not be better applications for alternative fuels than cars.

Electricity is already almost independent from oil. Vehicles are still almost completely dependent on oil.

The answer to your question is 100% (fear) and corruption. Project Majestic 12 have all the require answer to the entire field of home, cities, automobile, Airplane, Ships, Train and anything you can even imagine...However, no one single bank in the world or even some very rich people will support such project do to fear. They preferred to support the rich man toy name TOKAMA in search to replace the Sun someday.  Majestic 12 can be at any location in less than 18 months providing cities and industries with electricity, heat and desalinated water.

I think Achilleas is very much onto the right answer here.  If you look at the US GDP over the last decades something like 2 to 4% of it is related to transportation.