Good point. But I'd argue that's an artifact of history. Traditional banks started out when eventual consistency was the only choice. First with humans manually reconciling paper ledgers based on messages from couriers and telegraphs, later with the introduction of computers in batch processes that ran over night. The ability to do this live is new to them, and thus their processes are designed to not require it.

Financial institutions set up in this century (like paypal or fintecs) usually do want consistency at all cost. Your risk exposure is a lot lower if you know people don't spend or withdraw more than they have.