There were no MacBook Airs with Retina Display announced today on Apple’s special event.
The reason is pretty clear looking at what Apple had to do to develop the iMac with Retina Display. From Apple’s website;
A more advanced timing controller.
The timing controller, or “TCON,” is the brains of the display — it tells each pixel what to do and when to do it. Because iMac with Retina 5K display has four times as many pixels as the standard 27-inch iMac display, the TCON had to be able to handle more information than ever. But even the most powerful timing controllers available couldn’t manage this number of pixels, so we had to create a new one with four times the bandwidth of the previous-generation 27-inch iMac — up to 40 Gbps. Now a single supercharged chip beautifully orchestrates the symphony of all 14.7 million pixels.
To develop Retina Display hardware that has at least comparable performance to their non-retina products, Apple has to develop new hardware down to the semiconductor level. This hardware has to have processing power that is multiple times faster than previously available.
This takes time and effort. And as is obvious from the use cases, the iMac has much higher priority for Retina Displays than the MacBook Air.
For a bit more information on how pixel-density can adversely affect performance, here’s an article describing the poor performance of brand new Android devices with high-res screens.
Some Chromebooks have high-res displays but they have it easy. Nobody expects high-end graphics performance from Chromebooks, and there are no apps that take advantage of the performance even if it existed.