I know the theory—why can't I tune a real system?

You can recite the equations. You can draw the block diagram. And still the real system oscillates, or crawls, or does something the math never predicted. This is not a failure of your understanding. It is the ordinary distance between a model and a thing that actually exists. Here is what you are doing instead of tuning: you are studying more, hoping the next book closes the gap. It will not. No amount of reading substitutes for the hours of watching a real gain change a real response. You already know this. You are avoiding it because the real system is messy and does not reward you the way a textbook problem does — with a clean answer at the back of the page. The control loop does not care that you understand control theory. It responds only to what you actually do to it, today, with your own hands.