This seems a problem in several places. It would be better if tap on tap off was a lower price than buying a ticket from the driver.
People aren't used to it and many drivers don't encourage it.
Combine that with the fact many tickets don't cap with tap on tap off (only works for solo adult travellers and doesn't cap return fares so you pay more). For multi operator schemes, there is generally at least one firm which doesn't have the full capabilities. Plus, I think there is also a lack of trust, I know that I, as a solo adult traveller, tend not to trust the schemes too much as I am too sceptical about whether or not it will cap properly. Definitely for multi operator schemes, I don't trust them at all and depending on the number of buses you get on, the dwell time could be longer than buying a ticket because it's tap on, tap off, tap on, tap off versus one off payment then a very, very quick QR scan on all subsequent routes.
Why do so many companies use these Ticketer machines to implement tap in tap out?
Because for all of their faults, Ticketer is the market leader because of contactless in the first instance, and now bus open data. There are so many issues with Ticketer but quite frankly, they don't give a stuff because they are backed up by so many contracts, they needn't care anymore and they know that the cost to move away from them (in terms of admin work and actual cost of new machines etc), is so high that why would anyone change to another ETM provider anytime soon. There is no incentive anymore for them to do well so they are focussing now on how to get more money out of operators with the endless add ons they offer, rather than making the bus passenger experience better. They see it as 'bus company pay us more money' rather than 'please passenger who pays bus company so bus company has the money to pay us more money'.