>Maybe making cameras is really hard?
Making cameras can't be
that hard - every cell phone manufacturer puts at least
something moderately decent into their offerings, and Canon, Nikon, etc make digital cameras across the quality spectrum
>Cameras are too much of a commodity and the market is really saturated?
For most people ... why would you want YADTCA when you have a "good-enough" camera in your smartphone? If you can't take "good pictures" with your smartphone, you won't take pictures any better with a stand-alone camera - the camera is already better than you are (for 99% of camera users)
>Full size lenses are hard?
Doubtful - if anything, they're easier than miniature lenses
>Not enough profit margins?
Meh - you can make the margin [almost] as large as you want ... so long as customers will pay your price
>Maybe it would cannibalize iPhone sales?
Again ... doubtful: if you want a separate camera, there are already fantastic options out there. And, back to a previous note, the camera in your smartphone is already better than you are
Apple basically quit doing anything to the iPod line because the iPhone, quite frankly, is a better device (even if you don't use it for a "phone") - storage, features, processing power, etc
If Apple were to venture into the standalone camera realm, it would have to be monumentally better than what they can already pack into an iPhone or iPad