I just don't think it's particularly economical for them to include such a chip. While surely they'd get the chip at a nice price, they'd have to spend millions in R&D redesigning the motherboard in its entirety (I'd assume at least, since there's not much space in there and you need to shove antennas somewhere).
I really just don't see the demand for it... and for those who do demand it, surely they have a phone capable of tethering. As a student, I'm constantly surrounded by Wi-Fi networks (home, class, library, dining hall), so for me, there's really no need (which is also why I opted for a Wi-Fi iPad 2). If there does come a moment in time where I need to check something on the go (like when I'm walking to class or on a bus), I simply use my iPhone. Going along with that, if I actually do need to use my iPad or laptop in such an event, it takes 2 seconds to enable my iPhone's hotspot.
My laptop right now actually has a built in Verizon card. I've never activated it because it simply costs too much and have never had a dire need for it.
Meh, that's what I think at least. Just not a big enough demand.