It is kind of odd that such a simple task as returning unspent balance back to reserve requires a worker. Aren't those smart contracts not smart enough even for this?
They are. If no worker is voted in, the unspent budget would be returned to the reserve.
The thing is, if there is no refund worker, anyone can create a worker for themselves, vote for it, and it would be accepted. The refund worker creates a barrier you need to overcome, if you wanted to do that.