Because it's a prototype without noise filters or any other attempt to reduce motor noise. His point in turning off the motor was to show that there's little noise caused by the blades chopping through the air - which is where fan noise actually comes from.
If I had to guess I'd say it's because the video is targeted at people for which this will be obvious. They aren't looking to sell this directly to consumers. They want to license the cooling technology. As far as they are concerned, the noise of the motor is probably as irrelevant as the bracket's color.
I think what you actually hear is coil noise from the fast switching of the current in motor's coils. The motor they use looks and sounds more like a HDD motor more than a normal fan motor - optimized for constant speed under quite a heavy load. The fan motors work in a similar way, but they use less power and less coils in a bigger enclosed space, so they can be better optimized for sound.
In a production version the noise is largely caused by the air flow, whereas the motor noise would be cut down/eliminated by casing which is not present in the development version
