There are tons of automated systems which take cash. I do agree it would add some complexity to the equipment in the car, but its not like it would be that difficult to do it. These cars have to go back to the depot eventually, emptying the cash bins would just be one more cleaning step. I do agree though they don't bother because it probably would cost more to manage in the end than the amount of rides they're losing not accepting cash.

Just another example though, while credit card processing fees aren't always cheap its not like its free to process cash.