They don't, but most people with 17" or 19" CRT run at 1280x960, so that's what they make the LCD's at. 21" LCDs and up are usually 1600x1200. That's why I'm not getting an LCD, I like 1600x1200 on my 19".
They should try 1400x1050 as an intermediate res.
But, as I said, they are that res because it's the most common for that size of screen.