Apple also never said what the color resolution of the iPhone screen is, athough I'm pretty sure it's a really decent 24 bits.
I think part of it is what others have said: Apple doesn't want to get compared directly against any other device, if they can help it.
Especially now, with WVGA OLED screens popping up everywhere, some with 1 to 1.5 GHz Dragonfly processors using half the power they did before.
Besides, doesn't it feel more luxurious if the specs are meaningless to the owner? 🙂