I imagine Apple are optimising for battery life and cost here, rather than for sheer resolution.
Edit: Also, that's the 11", I doubt they can squeeze many more pixels in there before it becomes unreadable. The 13" is 1440x900, which is more reasonable (I'm using it as we speak, actually!)
This argument won't stand. New phone displays have significantly lower battery consumption than the old ones and substantially higher resolutions. And you can always employ tricks like reducing refresh rate to lower the consumption even more.
New phones are also not tied to the x86 platform, and generally run software that has been optimized from the start for performance.
There are other considerations too. Phone usage patterns are way different to computer usage patterns. The vast majority of phone usage occurs in small blips of activity during which execution can be optimized for the CPU to race to sleep state as soon as possible. Computers don't have the same usage patterns at all, and have a legacy of first and third party software (on all major platforms) that have not yet been optimized for performance.
Why not? If they push the resolution of the 11" much more, it'll become harder to read the screen. If they double the resolution to make it "retina", the battery will take a hit, as will the cost.
I'm still quite happy with my late 2010 13' Air, as a second/travel/coffee shop laptop.
About your edit: remember that how Apple does it in the "Retina" models is increasing the definition, without affecting the actual size of the UI elements.
People are defending the 11" Macbook Air with that resolution, but you make a fair point.
I have the 2012 model, and it is an absolutely superb machine, but the lack of vertical pixels is its weakest feature. Reading documents is far from ideal: it is distracting to read a wide-columned pdf with a relatively small number of lines on the screen, and the screen isn't quite good enough to allow much zooming out. There is a noticeable difference in the usability of the machine when my eyes are tired. A higher-resolution screen would presumably help on both counts.
Because the software is generally well-put-together and the touchpad is excellent, the resolution is not so much a problem for actively using software, creating documents and so forth. But scrolling around and zooming in and out has an overhead, which again, more pixels would reduce the need for.
This is their cheapest option and has an 11" display. If you want pixels then you should pay a little more and not complain when it is a little heavier:
They could differentiate by making the MBP a lot more powerful than the MBA with things like a high end quad core CPU, up to 32 GB of RAM and a fast Nvidia GPU.
Admittedly the market for high end powerful 13" laptops might not be that big.
Edit: Also, that's the 11", I doubt they can squeeze many more pixels in there before it becomes unreadable. The 13" is 1440x900, which is more reasonable (I'm using it as we speak, actually!)