Cameras are so common that it's almost impossible to buy a laptop or phone without one. On a desktop machine, it's still a third party extra. Why is that? Cost? How much more does it cost to put a camera in a phone? Low demand? There are a lot more people on Skype now, so demand should be rising. Customisation? If we build cameras into monitors, but you want a better one, you could always buy one.
Mokalus of Borg
PS - Those are the only objections I can come up with.
PPS - I'm sure I could find a monitor with a camera if I looked hard enough.