Because it's at least an order of magnitude more than what you'd pay in Europe for the same thing which is a good indicator that there is price fixing involved.
The key idea for SMS was to use this telephony-optimized system, and to transport messages on the signaling paths needed to control the telephony traffic during time periods when no signaling traffic existed. In this way, unused resources in the system could be used to transport messages at minimal cost.
So, in a sense sending SMS doesn't have to be so costly.
500 messages a day costs < $0.07 total. It's cheaper than that in Europe?