Why don't Car makers, TV makers, Watch makers, Clothing designers, house builders, etc etc, all just make 1 model then?
Might is be that humans are not all robots, we don't all act and think the same and have the same likes, dislikes, needs and wants.
Piggie... I kinda get what you are saying... but in the comparison you are picking more macro comparisons and comparing to a micro situation. Apple is a computer/tech company, thus they produce a wide variety of products from the Mac Pro, Macbooks, and iPads... etc to speak to different segments of the population...
Now if you are speaking about the content... that a different story and the above comparisons do not quite fit in this micro situation. Even though the usage of the iPad may seem to be limited. But I am sure my use of the iPad is drastically different from you and other people. Thus the iPad is speaking to a wide array of people for different reasons. But Apple is trying to limit the experience to certain facets, so unfortunately some segments of the population will be unhappy. Only time will tell how this approach will work in the long term.