Path dependence. The problem is that, if just one restaurant does it, a lot of people see the higher prices and don't realize a service charge and sales tax is included at that one particular restaurant. It's not unheard of to do but it's very uncommon and mostly doesn't work out well. (Plus both waiters and customers are mostly just used to the current system.)

This is starting to happen, albeit slowly. It's tied in to a push to equalize income between the back and front of the house. In the currently-prevailing model, back of the house is totally screwed money-wise relative to the front.