That's a major oversimplification. Car dealers can provide a lot of value to individuals looking to purchase a vehicle, both at the time of purchase and after the fact via test drives, financing, and discounts for future maintenance at their service centers. There are plenty of shady car salesmen that take advantage of vulnerable people, but that should not negate the benefits that many dealerships provide.
The stock issue is tricky, as many local dealers can't afford to carry a wide range of models and configurations. Stopping by the local dealership looking to buy a specific model vehicle same-day may result in disappointment. However, dealerships are very good about acquiring a vehicle that you are interested in purchasing. All you need to do is ask.
That being said, I agree with you that buying a car should be more similar to buying an iPad than it currently is. There are ludicrous laws that stifle innovation in the vehicle purchasing process, such as the Massachusetts Franchise law 93B[1] that prohibits manufacturers from owning dealerships. I think Tesla's vision[2] for first party dealerships and vehicle distribution is a step in the right direction.
[1] http://www.malegislature.gov/Laws/GeneralLaws/PartI/TitleXV/...
[2] http://www.teslamotors.com/nl_NL/blog/tesla-approach-distrib...