Damn, good question. I've been wondering the same thing. I think pmbuko said it best:

"The fact is that the aspect ratio of TVs in peoples' homes is the LAST thing on a director's mind when choosing what size film to shoot his/her movie on."

Either that, or it's marketing. They want us to stress over all the details. Pretty soon they'll start selling TV's at all different kinds of ratios. Damn them, damn them all. Juuust kidding.