All of this is speculation on my part since I don't work for Apple or have any access to someone who does.
1. Apple has the iPhone, the iPad, and the iPad mini. This works out to 3 models of mobile device.
This is probably enough differentiation on a practical use level.
2. Most companies that have many different models of desktop or laptop are are targeting specific price points in an attempt capture more of the market. It's commodity computing.
Apple doesn't seem to care about price points or offering a commodity computing device. They offer basically 2 models of laptop and 2 models of Desktop. Each is widely considered to be top of the line for their particular category. Apple doesn't care of the middle or low end for those categories. This removes any incentive to ship multiple models.
In short Apple's game is not to be a commodity producer but a Luxury Good producer. They ship exactly one model for category of computing device and that device is intended to be the best in class for that category.