zero index makes sense because (and i will use C syntax here)

a[b]

is equivalent to

*(a + b)

with a being a pointer and b an integer. this can be seen as well in the fact that

a[b]

is equivalent to

b[a]

where in both cases a is a pointer and b is an integer.

Thus, the index into an array is fundamentally the same as an offset into that array. Usually, in languages that do use 1-based indexing (like in pascal strings), the 0th byte is used for something else.

The point being that thinking in terms of 1-based indices means that you have to use a system where addr+0 is invalid, and the first element of an array at address X is X+1.

Now you could just shift everything by 1 and just declare that +1 means +0, but then you lose all meaning to the offset vs index and ahhhhhh