Why? Well for one thing, there is the size factor. Laptops are generally smaller than desktops. When using them, you cannot shake off the feeling that you are being cramped.
And then there is the screen factor. The screen is smaller but then we still want to have a lot on the screen so the pixels are smaller. Especially the 12-inch ones that attempt to cramp 1280X800 pixels onto their tiny screens.
The keyboard? Well, the keys are cramped too and the big buttons like the Return, Shift and Space Bar keys are shrunk. Probabilities for typos are maxed out and this means lots of frustration. If you are working using a laptop, chances are your work will be finished much slower.
At least the 14-inch ones with more ergonomic features are a lot better. I have been using my 12-inch NEC for the whole of 2009 and believe me, it was close to hell. I switched back to my old big Acer laptop and life was so much better. Then for 1 whole day, I used the 12-inch NEC again, and the stress came back. I felt more tired, my ankles swelled and my belly flabs came back. Stress and Strain makes an unhealthy man.
My recommendation is to NEVER use a laptop as your primary workstation. Just use laptops as a means of accessing the Internet for critical information while you are traveling or away from your home PC. Laptops cost so much more too... dang.