My undergrad degree is in electrical engineering — not physics — but there is a good deal of overlap in the subjects studied while pursuing either degree. The difference between the two fields is that physics is concerned with discovering the laws of nature, while engineering is concerned with applying them to solve problems.
All metaphors sooner or later break down, but it occurs to me that competitive shooting can be more or less regarded as the physics (science) of shooting, while defensive shooting is the “engineering” discipline that takes the efficient techniques that competitive shooting discovers and applies them to solve real-world problems.
Now a self-defense situation is not concerned with efficiency but rather with effectiveness, although sometimes – not always – effectiveness comes from being efficient, and efficiency is always good. Just as an engineering solution may not be (or may be) the most elegant solution from a pure science point of view, but may be “good enough” given the other considerations that the engineer (and their employer) have to consider.
Further, usually self-defense effectiveness is heavily dependent on things other than shooting altogether (such as tactics, awareness, legal considerations, verbal skills, etc.). Just as an effective engineering solution has to also consider, besides the science of the situation, the other real-world factors of economics, distribution, backwards compatibility, and existing technology infrastructure (to name but a few).
The end result is that defensive shooters can learn a whole lot about pure shooting from the competitive world — and should. But no one should confuse shooting competition prowess with the entire toolbox needed to be effective in street encounters.