Because a good bit of people don't want to pay $1800 for an underpowered system whose specs are pretty much locked up at the factory settings. For $200 more, you could get a MBP, or for $800 less, you could get the WhiteBook, or for $500 less, you could get the base uMB.
And yet many people buy or are going to buy the MBA, like me.
Why? Because it's not all about specs or price...it's about ultraportable thinness/lightness linked with reasonable performance on the road...a MB feels much heavier than the MBA, and the same happens when the comparison is made with the MBP. Besides, no other notebook has that with the industrial design edge that Apple offers.
Unfortunately, most PC makers have no clue about that, and keep presenting fugly machines that have 174-1 card readers or whatever...99% of us don't need and have never wished for these kinds of crap; go figure.