For the same reason people who steal credit card numbers sell them rather than use them, why illegal drug distributors don't sell to users directly, and mob bosses don't rob people: They can probably do the same thing at the same time with multiple people, and it reduces their risk exposure. If 10 different people open up a bank account with stolen credentials, the bank has to detect fraud 10 different times. If one person opens up 10 bank accounts with stolen credentials, they're way more likely to catch onto a subtle pattern.