Is control theory too abstract?
For most real-life applications (read 90-95%), a PID (proportional,integral,derivative) controller in a feedback loop is sufficient. Although there is a lot of research done on things like self-tuning controllers and advanced theory for specific applications like flying supersonic aircraft, a high proportion of controls problems can be solved without many of the advanced techniques. 

It can be the case that the calculations for some control algorithms is too complicated for the computer involved, but oftentimes the main reason that engineers stick with PID is that the improvements from more advanced solutions aren't needed- they might only need to hand tune the controller gains once and then the system response is good enough that they can forget about it.

Source: I'm in graduate school for mechanical engineering/robotics.