One reason is unstated.
In order of costs per transaction, with an order of magnitude gap:
Branch network.
Human-assisted telephone banking.
ATMs / automated telephone banking.
(Bank by mail barely relevant to this discussion due to relative volumes.)
Oh that's a thing. The overwhelmingly most common use cases are depositing checks and opening accounts. (Historically used to be huge in credit cards but, thankfully, online account applications are eating most of it.)