I'm not absolutely sure, Paperman, but I think it comes down to physics why they're close to the center. In order to get the parallax measurement, light from both sides of the lens has to get to the point you want to have a focus sensor, and on the edges of the image you don't get light from both sides of the lens, or at least not with some lenses.
This is at best over-simplified and possibly wrong, but it's the way I think about it. I only dared answer because you asked if anyone had an 'opinion' about it. ;-) I think technically you actually have to have light from both sides of the lens hitting prisms just before the sensor, each of which selectively directs light coming from one specific part of the lens to the sensor, but I think it works out to be similar to what I described before.
So to answer your question, I'm pretty sure they could have AF points further from the center for some lenses with a slightly larger image circle (PC lenses, for example). They should also definitely be able to put them as close to the bottom edge of the frame as they can to the side edge of the frame. I'll bet someone has done a cost-benefit analysis and it's not worth the extra design and production dollars because not everyone will use them or will even have lenses that would let them use them.