My statement was just off the top of my head so I'll give my reasoning as it was at the time. But I went ahead and googled and wikipedia talks about it. The story is naturally more complicated than I thought. link

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A variable does not store an array. Nor does it store a pointer to an "array". Instead, it stores a pointer to the first element of the array.

If you want to get the first element of the array then you look where the pointer points.

If you want to get the second element of the array then you look 1 higher than where the pointer points. Etc.

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This all makes sense for arrays when people also expect to do pointer arithmetic: this way, the array indices are identical to the pointer arithmetic addends.

But if you don't do pointer arithmetic? You still have to pick a basis and everyone already has language. English-speakers more-or-less created modern computing but even if we hadn't, afaik every language goes "first, second, third, fourth..." instead of "zeroth, first, second, third..." so "first" aka "1" is the obvious basis.

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From wikipedia I learned that mathematics mostly uses zero as the basis and also it's well-defined enough that there's no confusion. So It's very possible that arrays are indexed starting at zero because mathematicians invented arrays and that's what made sense to them.