Why are we still using gasoline as a fuel source for automobiles?

Simply put.

It is currently the most "Cost effective" means of propelling a vehicle. When you add in the "Life time cost" of the vehicle, the cost of the fuel, the cost of parts, and the cost of disposal; internal combustion engines just beat anything else. There's really nothing else comparable. Not yet.

Electricity will eventually be a viable source, but current batteries only get us about 100 miles. Current fuel tanks get us 300-500 on average. And the cost of a "comparably sized vehicle" is half, and the recouping of your fuel costs will take the life of the vehicle.

There are two simple reasons; lack of better technology, and lack of funding for said better technology. Electric cars would be great, but until there are batteries that can carry a car as far as gasoline can, with fast enough charging times, than people are going to choose gas.