Of course Apple could have fit a 13" screen on the smaller Macbook Air, but if they did this they'd have to increase the costs of the smaller model, plus they'd have to release a 15" air as the second model also with an increased costs, as nobody would buy a larger model with the same 13" screen size.
In the future when Apple releases new versions of Airs, perhaps they'll release a model with a 13" screen in the smaller form factor currently known as the 11" model, and bump up the current 13" model to 15" screen. You can't expect Apple to do this now as it would mean less revenue for them.
Anyway, I don't find the bevel on the 11" model to be annoying at all, as I spend my time focused on the screen, and not the bezel that surrounds it.