What if IRS doesn't have _all_ of our tax data? Then people would go like "but IRS told be to pay X, it shows that they don't know about my other source of inc

What if IRS doesn't have _all_ of our tax data? Then people would go like "but IRS told be to pay X, it shows that they don't know about my other source of income, so I better not tell them". In other countries with automatic payment (India, Ukraine) -- people would not disclose _any_ incomes for taxes, if given a chance. So government makes everyone to pay through a monitored taxed account. But people don't even think that if money come on another, non-business, account, they should disclose that as income. It's kinda assumed that if gov doesn't know, then they shouldn't pay.