For the price they couldn't jam it in this unit... And given most (Probably 90% or more) wouldn't use it anyways on this first go around. I bet next year if not sooner you will see it pop up. You can always get one of those little hot spot devices and get a case that can hold both... More bulk but not too bad of a solution.
It will be years before you find one. Cellular radios and the supporting hardware are expensive. You'd be looking at a minimum of $100 price increase if not more to pack one in a tablet. You can either get an expensive tablet with a cellular radio or a cheap one without it. It's just not possible to do a sub $300 tablet with cellular radios right now. Just go get a wifi hotspot if you're concerned about phone battery life. Most are free with a contract.
More so than the price, I think this was a huge factor. Whenever there are carriers involved, Google can't seem to do what it wants with the device however it envisions the device to be. Especially with CDMA carriers. This allows Google to offer the complete Google experience without compromising anything (other than cell data accessibility). The next OS update will likely be released immediately to the N7 and this is in big part because Google doesn't need to ask anyone else for permission or what not. They can just push the update out and be done.