ok heres the thing. people who buy laptops generally are willing to pay a premium for those higher resolutions. thats why theres the option to get a higher resolution screen.
the 17-19in standalone lcd monitors are made for mass market. they make them cheap because the majority doesnt want to spend so much. its the bottom of the barrel so to speak. its all about money, and i'm afraid theres probably not too many wanting to buy a 17inch lcd with 1600x1200 resolution, nor pay a high premium on it. a company would rather sell 10,000 $199 monitors.
also, you cant just buy some laptop lcd screen off ebay and put it in a plastic enclosure you find. i've looked into that once upon a time, but its more trouble than its worth, if not impossible. basically the lcd screen has this little connector that is specifically made for whichever laptop they are built with.