Bitcoin transactions in which the payee retains the currency instead of immediately exchanging it for USD, EUR, GBP, or another usual currency are largely done for illicit purchases, especially those involving narcotics.
This is done because Bitcoin is extremely easy to launder through "tumblers". Unlike, say, paper currency, an individual 'unit' of Bitcoin (insofar as that exists) is not tracked with a serial number.
Additionally, in the US, the IRS is underfunded and prolific sellers are smart. Unlike banks, Bitcoin does not inherently have names attached to accounts/wallets, and so it is much easier to evade taxes than with typical currencies.
This is done because Bitcoin is extremely easy to launder through "tumblers". Unlike, say, paper currency, an individual 'unit' of Bitcoin (insofar as that exists) is not tracked with a serial number.
Additionally, in the US, the IRS is underfunded and prolific sellers are smart. Unlike banks, Bitcoin does not inherently have names attached to accounts/wallets, and so it is much easier to evade taxes than with typical currencies.