Most public buses have a vending machine type payment system. You can insert bills & coins, but there is no outlet for returning change. So, there is no way drivers can return change.
Which is still stupid, because vending machines have had ways to return change for decades. It's just bad design.
Fare boxes don’t vend change because it is another moving part that can break. Waiting for change creates dwell time multiplied by cash boarding. Agencies can buy fare boxes that give change but this only makes sense on lower ridership systems.
All trams in Berlin have ticket vending machines that give change. And Berlin definitely doesn't fit the "lower ridership" category.