The history of why the car dealership system exists as it does is actually quite interesting. Back in the day, car dealerships were the good guy underdogs, and car manufacturers were pretty much the devil. The "Planet Money" podcast has a great episode on this:

http://www.npr.org/blogs/money/2013/02/12/171814201/episode-435-why-buying-a-car-is-so-awful

Basically explains why buying a car in general sucks (consistently ranked as one of the worst consumer experiences), and why there isn't a "new car supermaket" where you can browse & buy cars from multiple manufacturers.