Because, in an economic sense, of asymmetry between the parties. You only have a handful of car companies to buy a $30k green 5-door sedan from. But Ford can easily afford to lose you as a customer - Ford is more important to you than you are to them.
An extreme case of this (in some places) is the internet provider. If they know they're the only ones they can basically charge much more than if you were living in an area with healthy competition. What're you gonna do, not use the net?
