Another answer: because regular banks are awful to deal with. They don't have nice APIs to just move money around. They apply velocity limits at surprising times. They can terminate your account suddenly for opaque reasons. They charge high regular fees and even higher "gotcha" fees if you make a mistake. They can take back money sent to you for up to 90 days, but often can't get back money you sent to crooks even if you discover your error the next day. And 100 other complaints.
So if you're doing something involving moving a lot of money around, you may need to be your own bank to make the whole system work well.
So if you're doing something involving moving a lot of money around, you may need to be your own bank to make the whole system work well.