I'm not an expert in controls, but it's an area that interests me and I've taken a couple classes. It's true that the theory of linear, time-invariant systems is pretty complete. In that very limited framework, you can basically make a system behave in any way you want. The interesting thing is all the various infuriating ways in which real systems fail to be linear. Those nonlinearities can be accounted for, but now you no longer have a turn-key solution. Furthermore, there really is no such thing as the optimal controller. It all depends on the objective function you are trying to optimize. How much do you value a fast response time versus strong disturbance-rejection? Would operating the system that way cause undue wear and tear over the service lifetime? Would it cause danger or discomfort to human passengers (if any)? How sensitive is system behavior to uncertainty in model parameters? It's obvious that the design objectives involved in building an air-to-air missile are very different from those involved in building an elevator. But explicating them and translating them into an actual controller requires a great deal of care.