If anything, 1400x900 on the new MBA seems a little bit small (and Apple doesn't allow you to bump the size of everything up like you can in Windows). It's interesting that 1680x1050 is still an upcharge on the 15".
Because people want higher resolutions without using an external display. Or at least a choice.
The best I can tell, the availability of high resolution screens on laptops has actually dropped over the past few years rather than increased. It seems rare now that screen resolution is even an option on laptops.
Personally I'm one of the people who is in the market for high resolution laptop screens. Unfortunately its a choice between high resolution and durability or size. I'm never traveling with a 17" screen again and I don't really want a poorly engineered Vaio Z.
If Apple put in a 1900x1080 screen, what you'd hear instead from consumers is that everything is too tiny to read.
In other words, the people willing to pay Apple prices care enough about their computer purchase to actually read the specs. Like if you buy a high performance car, you at least want to do enough research not to look like a total idiot in front of your gear head friends after you've spent that much money.
Or maybe I'm wrong, and Apple customers really only care about how nice it will look on the coffee shop table.
My MacBook Pro 15" from a couple of years ago has a 1440 by 900 display and that is MUCH better than this 1280 by 800. Shame that Apple still thinks that is acceptable.