Like I mentioned before, it doesn't fit the typical usage case.
Maybe it should be offered as a BTO option for MBAs, although having to make the necessary design changes to accommodate the radio/antennas, an external antenna band, and a SIM card slot probably isn't worth it for only a small group of non-typical potential buyers who refuse to tether to their smartphone or don't have one with a good data plan.
There are a million reasons why Apple might not want to add this feature to their laptops. I don't know the actual reasons but it's probably a combination of some/all of these:
- Maybe there's no good spot for the necessary antennas in the rear hinge, or there would be EMF conflict with the antennas that are already there.
- The chips necessary for cell communication take up a small but non-trivial amount of space and maybe there isn't enough space on the MBA's already-pretty-dense motherboard.
- Maybe there's a software/IP component to the cell stack that Apple would have to license, and they clearly wouldn't want to license it for every copy of OS X, nor would they want to have two different versions of OS X floating around, one for cell communications and one not.
- Adding a one-off feature like this increases manufacturing and distribution complexity. Two different versions of the motherboard, different unibodies (one with a SIM tray, one without), different FCC/EU certifications necessary, etc. Clearly Apple can do this because they do it with the iPad, but if they don't expect many people to buy a cell-enabled laptop, then it might not be worth the extra cost and complexity.
- Maybe they (or the cell service providers) think that if the laptops don't have cell capability then people will be more likely to buy iPhones and tethering plans.
Anyway, it's all kind of pointless to speculate.