1-indexing in music is as much a mistake as 1-indexing in programming languages. The traditional interval names are mostly 1-indexed too, which just adds pointless complexity you have to compensate for whenever you're adding or subtracting. Only exception I can think of is the tritone, which is inconsistently 0-indexed (and in a different scale).

It's IMO crazy that going up seven scale degrees makes an "octave".