Banks don't report every transaction and even if they did it may not be "income". E.g. moving money from one account to another is a transaction but no income was earned and it is not taxable.
Surveys show as much as half of workers make money outside normal employment so this is not some obscure side effect but the common case.
[1] https://finance.yahoo.com/news/side-gigs-2023-50-americans-1...