Because there is most likely something illicit going on. I thought this is perfectly apparent. Sounds fishy as fuck.
Not sure about prepaid cards, but surely many broker must be accepting that. What you can do is to link your cards with paypal and then use that as funding method, that can work?
Those prepaid debit card is like Visa gift card. Broker can draw fund from it as a purchase, but not as an ACH transfer.
You might find a broker that does that, but there is good chance they will limit withdrawals to the same card it was deposited with.