And yet it isn't. In all the years I've been shooting, I can't remember anyone complaining that it really hurt them. On the other hand, restrictive bondage, toe-ties, pointy nails...especially on armpits!
The hairbrush is easily the most effective tool overall, especially when it's used alongside fingers etc to create different sensations. It doesn't work on everyone (mainly because some people find it a pleasant sensation), but it does really tickle for most people, and it helps to ramp the intensity up. That's why it gets used a lot - not because people think that hairbrushes look particularly great.