Most likely reason is to avoid drivers being robbed - if there's no (easy and accessible for the driver) way to take money out of the thing, local addicts won't get an idea "let's jump on a bus and take money from the driver" because driver can't really give the money.

Also a bit of avoiding fraud from driver's side - if the driver is allowed to take money out, it's impossible to control whether all the payments actually got to the end point.

That said, I've seen systems where the drivers give change - e.g. in Israel - and that's wasn't a big deal. So I guess it also depends on the locality.

It's also to speed things up. Many drivers want to hit their timetable; having to give change to 3 or 4 different people per stop is going to kill that.

The European busses solved that ages ago.

Driver presses the button for a special ticket, customer inserts coins or bills, the machine automatically gives exact change. Alternatively, pay with NFC, card, etc.

Or they have a small cash register and just give change back to dozens of people in the line, I've never seen a bus being noticably late because of it.

Honestly some of the things mentioned in here are mind bending. I have public transport experience from both London and a small country on the eastern side of Europe, and both were efficient, fast and safe. What the fuck is wrong with the US when people are scared to take public transport because they can get robbed?

Well, in some European countries the drivers do not handle any cash or tickets. Both for safety reasons and for the fact that having a queue of people waiting to buy tickets from a single driver introduces latency. Instead, there's a ticket machine on the bus stop, or sometimes inside the bus. Alternatively you can buy a ticket using a mobile app. This means you can board without a ticket. Then ticket inspectors randomly board vehicles to check peoples tickets.

There are vending machines that can give you change back. They should just pop some of those machines into the bus entrance.

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.