Mainly because of the problem space. Banking/finance is a complex field with many moving parts: card processing, validation, encryption, currency exchange and t

Mainly because of the problem space. Banking/finance is a complex field with many moving parts: card processing, validation, encryption, currency exchange and tons of certification stuff (I believe Nubank's primary focus is card processing). Didn't even touch other financial fields like investing, credit handling, or any kind of trading. The software is always expected to be correct, never fail and fast. Otherwise, someone will lose the money, and usually, you (bank or institution) will have to recoup that, plus penalties, plus name damage. Source: worked there.