Apple has not yet offered Retina displays on Macbook Air because of two reasons.
1. Product Positioning and Price Points
Skimming the professional market first as the displays are only produced at low volumes first and therefore focussing on a market segment that is less likely to turn every dollar before it is spent. If successful, then the volume goes up and manufacturing costs come down and the product becomes viable for introducing in the mass market = Air's.
2. Battery chemistry and life.
High resolution displays draw more power than lover resolution because there are more transistors per panel that need to switch. One example is the iPad 3 Retina, vs iPad 2: while almost offering the same run time as the iPad 2, the iPad 3 has a totally different battery system and capacity. Fully charging an iPad 3 from almost empty to full takes a full night, much slower than a full iPad 2 charge cycle. While that works well for the usage pattern of a tablet, it is not adequate for the usage pattern of a notebook that draws much more power.
If you charge a LiOn or LiPo battery with a too high current the battery explodes (some impressive videos on youtube). If you charge such a high capacity battery with today's technology of a safe charge current then it takes ages to a full charge.
The MacBook Pro Retina is bigger and therefore has more space for battery, which is a good workable compromise between battery capacity and charge current.
As a rule of thumb a standard rechargeable battery reaches the highest life time if the current to charge it is about the same as the current consumed in the discharge (that is not marketing, that is chemistry). So translated into hours of battery life (and demonstrated on the iPad3) a battery run time of 9 hours mandates a charge time of almost the same = 9 hours.
So I would love to have a Macbook Air with a battery run time of 12 hours, but I would not want to wait 10 hours to have it fully recharged again.
So here my call to action:
for all the bright undergraduate and gradient students out there:
Move to the study field of battery chemistry and you may make developments that truly change the world. Not just for MacBook Air's, but also for banking who needs better batteries to consume solar energy charged during the sunny day during the night.