Why do all those netbook need to have fans?

This is a question that I've been asking myself ever since the first eeePC was released. Sure, there are other problems like bad keyboards and mechanical design (with the exception of the HP mininotes) and too small / low-res displays. But the question that bothers me most is: Why do those supposedly-so-power-saving new processors still need a fan in those systems?

I am the happy owner of a Panasonic Let's Note CF-R5. It was bought in September 2006, where it was already end-of-life. It does not have nor need a fan, the Intel U1300 (1.06GHz) CPU and the Intel IGP chipset are doing quite fine without it. So why are the supposedly less power-consuming later systems like Intel Atom built with fans? It's really annoying to me. I don't want this kind of noisy design. It sucks. It is a clear sign of bad engineering.

So far, the only replacement for the CF-R5 that I would consider is a CF-R7 or CF-R8. A netbook faster than the CF-R5 without fan could make me reconsider.