You obviously haven’t done it both ways and are assuming that spearing requires more dexterity than cutting. Hilarious that you could just try it for yourself and figure out that knife in the dominant hand works well but choose instead to bore everyone with your ignorance and stunning closemindedness
Do you always get fish served deboned? Cutting it with non-dominant hand sucks, especially more bony ones like trout. For me doing almost anything with my non dominant part sucks, my left hand is 20x less useful.