What am I missing?
I wonder if they have problems with anti money laundering regulations? Financial transactions are supposed to clearly indicate the beneficiary. If you're paying into a wallet that gets divvied up later, that's not possible so showing the paper trail of who benefits gets difficult.
Now, since it's basically acting as an escrow payment, Patreon might have some laws restricting what it can do with that money while it's holding it. For instance, they likely cannot legally earn interest on that money. Similar to how the security deposit held for a rental cannot yield interest unless that interest is paid to the renter.
That's sort of the same thing here, right? Prefill a wallet and you don't have to pay extra for processing fees (equating to convenience going back to tollways).