Because the receiving account was either opened with a stolen identity (darks) or was simply compromised itself. Once the funds hit, the clock is ticking and the fraudster needs to quickly extract as much value as possible before the account is frozen. This is often achieved by using a connected debit card to buy crypto [1]. The "merchants" who take debit cards typically use merchant accounts opened under stolen identities as well, and make up for the fact the accounts get hammered to death with chargebacks by taking huge volumes at very bad exchange rates for the time they're able to. Another method is selling macbooks/iPhones ordered online to be delivered to local fences, who then pawn them off on Craigslist or eBay and take the risk.
[1] https://paxful.com/sell-bitcoin/with-any-payment-method/?pay...