Now that's a good question, my bad that I misunderstood what you meant.
My first inclination is to suggest that you can only work with what you have, and that changes to the calculation are always at least a cycle behind. They tried, but they have fallen behind.
My second inclination is that the desired range was determined as a means to justify rates going forward.
I'm sure there are other reasons, but I think that those--especially the 2nd--are the major ones.
As you suggest, measurement might be a problem, but that cuts both ways, without any good means of divining which particular way was dominant this cycle, or any cycle for that matter.
As for GDP growth because of population growth - sure that's a part of it, but remember that the days of population growth are over.
The US population grew just 0.7% last year and it is falling fast. Any slowdown in immigration, legal or illegal, will bring that even lower. And worse - the portion of elderly non working Americans is getting bigger and bigger every year. So those are two big drags on GDP growth.
This is bearish. Not clear how capitalism works in a declining population.