Because you can't just have one size to rule them all! Didn't anybody learn ANYTHING from Dark Lord Sauron's mistakes?!
You start down that road, and then pretty soon you will have all sorts of discord. The Dwarves will go all hermit, the Elves will run away to the forest to sulk, and the Humans will strut around like they own everything. (Nobody cares about the Hobbits ... filthy little buggers anyway, with those hairy feet and all.)
BTW, it's one of several theory's making its way through the forums. Another is that there will be a 16" MBP in the same footprint of the existing 15". Maybe that will help ease the "suffering" of the 17" camp.
Well, a variation of that theory is that the 16" will replace the 15", and the 14" will replace the 13". So, the same number of sizes will remain after the dust settles ... again, in theory.
Maybe when they are able to remove the bezel around the screen (as dell) they are able to fit 1 extra inch of screen real estate into the same sized computers?