There is an argument to be made that numbered spells are also traditionally FF since early localizations had to use numbers due to a very strict character limits of 4 on spell names, which led to numbers being used, you didn't even get Fire 2, you got FIR2 and you had to like it, perhaps that mood is what the localization team was aiming for.
https://i.imgur.com/dsK20bt.png
But I agree with you, the -ra/-ga are used in so many more games, and while -ja is sparsely seen, it's more that the final tier of spells is sparsely used period.
