Because it might be less expensive to spend some human agent time on processing the possibly small number of voucher payments than to invest into a possibly more complex change in the IT system? I don't know if the number if voucher payments is small, and I don't know how complex and costly the change in the IT would be, so I am not saying that this is the reason - but it very often is the case that it's not worthwile to spend tech money on improveing /automating a process which is used not that frequently.