By single I mean a fare for single journey as quoted in #20. If the fare is kept the same, the bus company loses revenue (as people will only need one single ticket instead of two, one for each leg of the journey. or a day ticket), otherwise they increase the price of a single ticket to compensate, in which case you will have passengers complaining that they are only travelling four stops so why has their fare gone up for a service they don't use. You can't please everybody.
A lot of Birmingham routes are radial so, if you are in town, there is no point going half way home as the bus you will have to wait for will have come from town in any case. There are some routes which aren't but they tend to be less frequent so the next one may not arrive within the hour. Also, there are a number of different operators - one main one and several others, including those that compete and those that travel in from adjacent counties - would the scheme involve them? All of this makes it more difficult to administer. I seem to remember there might have been such a scheme at one time (probably pre-NEx) but, if so, there must be a reason why it isn't about now, and I suggest that is simplicity.