Are you perhaps missing the point that the bike is fixed wheel? With no freewheel there are momentum gains that some riders love. I've also heard that thieves fall off them at the first corner!
If you mean BMX's it's because they're very light, explaining the cost, allowing for them to do stunts/jumps. Going to skateboard/BMX parks and learning the tricks to be done is fun and if you get good at it it does indeed make you look quite cool.
