because you choose not to....
Because unless I want to live in the car, I can't afford it. It's common sense really more than anything else.
Not everyone is able to buy iPhones and Macs, they simply don't have the disposable income to do so, so they buy something cheaper and this may cause them to dismiss Apple products as overpriced.
There have been countless threads in the Mobile forum where people have said "Well, I'd like an iPhone but I can't afford one".