If I had to guess why Apple designed it this way? Here goes: Apple can't determine what a resting hand is as an abstraction, because sometimes a resting hand should do something (e.g. the pinch gesture; one static touch + one moving finger). So app developers would each have to implement their own heuristics to decide what's "probably" an accidental touch and what's "probably" a deliberate (but sloppy) tap. Result: you won't be able to predict anymore how the iPad will behave. That's far worse than having a predictable but imperfect system.
Probably wouldn't help in the case of a toddler's hands, but, as mentioned, the hand-on-edge is something many have trouble with.
Boy was I wrong, and I was really surprised at how sensitive the thing was. After a couple minutes of the pointer jumping all over the place, I turned it back on, impressed with the algorithm that had been ignoring those accidental touches all along.
Of course, the software and hardware is calibrated to have a specific threshold, but the fact that technology is commonplace that can detect when something is near it is just amazing. A former roommate's Dell laptop did the same thing - the controls on the side lit up when your finger got near it, so you didn't have to fumble for the buttons OR deal with annoying LEDs all the time.
This is part of the reason the iPhone always felt so fluid to me. I can flick a list, barely even grazing the screen, and the list will spin like a slot machine. It gives you such an immediate sense of powerful, accurate control that it's very gratifying to use.