The payout cost is probably less about the bank fee and more about accounting and other managing overheads. Remember that steam may have to handle refunds that happened after the payout. Would be a mess to chase a few dollars from a dev that doesn't have a proper revenue stream. Just not worth it to steam.

Also, correct me if I am wrong, but I am pretty sure the money not paid transfers over to another month, so you probably get most of your money sooner or later.

It's not ideal but it is understandable IMO.