Apple‘s entry-level configurations start at „somewhat usable but sucks how small they are for anyone and anything other than the most basic of use(r)s“.
They always aim to make it really easy and convincing to spend a bit more on the next, bigger tier: An amount that’s moderate in absolute terms - but ridiculously overpriced relative to the additional costs Apple pays on components (e.g. sell a $5 upgrade for $50).
64GB storage just (still!) hits that mark perfectly, as in „Even if it’s just for grandma, maybe we should still spend a few bucks more to get her the 128GB. 64 s really low from my own experience, and t my devices…“.
128 GB doesn‘t.