Because music theory doesn't know how numbers work. The interval of A(440Hz) to A(440Hz) is "1" in music theory even though it's 0 in literally every other context ever in the history of everything. I mean, if they're going to use a bunch of mathematical terminology, the least they can do is count right.
It's not a "1", it's a "prime" and the root of that word is an ordinal (whence "first") not a cardinal. There is no zero'th ordinal. There is just "none", which implies that A to A is simply not an interval.
