Because they are noisy (the quietest are ~20ish db), and the 'bad noise' because laptop fans are small and high-rpm type, which makes the noise on higher frequency.
I tried few notebooks and netbooks and the fan noise is easily detectable especially when I'm listening to classical music with lots of quiet passages.
This is the reason behind looking for a silent setup, rather than quiet.
At work IT support knows me well because I am the reason behind cleaning up fans of PCs around even 5 meters away from me I also suffer a lot sleeping in hotels; unless the room is completely soundproof, my ears catches any noise (which my wife loves making fun of me about).
Edited by recca_cool - 7/2/13 at 3:52pm