FWIW, even in places like Estonia, which has a free public transport (bus only?) system, the tickets are still sold/used/checked. Reportedly to get usage numbers and to cost-optimize the routes.

Each resident/citizen can buy a public transport card, then tie it to their Gov ID and then tap it everywhere. You could argue that this could be replaced by some vision tech but I guess this is simpler and has dual use (visitors can purchase the card and pay instead of using for free).