They're not. I'm scrolling around the Dell website here - depending on which model you pick, I can see 1280 x 800, 1440 x 900 and 1920 x 1200 available on several of their 15.4" lines.
Now, the real question, given that Dell do offer such wide options ( especially on their more expensive lines ) is why Apple only sell 15" screens at 1440 x 900.