Are we missing the point that the Main Character, Goku himself is an Alien that his race never showed power fluctuations other than transformation?
Vegeta after after staying on earth learns that emotion and bonds affect power level.
So a learned ability then. Not moving to earth makes you magically a earthling. Like moving to America makes you magically an american
I put my heart and soul into my work and I fear I have lost my mind in the process