Fearless Tower
Touchdown! Greaser!
Simple answer: Tradition
Longer answer: the FAA registry does not purge pilots from the database. There are no accurate numbers of how many are active. The logic is that an IR rated pilot is more active than a PPL and therefore (must be) safer. A large number go on to become CPL and work 121/135 jobs, hence safer.
There is no conclusive data to say that a Part 91 GA operator in a SEL airplane flying for hobby is safer with or without an IR, same as there is nothing conclusive on SEL vs MEL airplane safety.
You really believe that?
You think insurance companies do that out of tradition?