If you buy a sandwich in Tesco and another of the same price in Sainsburys, you pay the same as if you bought both at the same place. You can have loyalty cards for both shops so you don't really lose out there either, other than that it takes longer for either to realise any benefit.
People will probably plan their long-distance journeys longer in advance, and will probably book a seat on a specific flight or train, so will select the operator that is most suitable for them and stick to it (or in the case of rail pay a lot more for a walk-on). With a lot of bus routes it's a bit random which bus will come next, and what might be high-frequency routes during the day probably aren't if you return home in the evening, so you might need to wait a long time for your chosen operator or pay more to use a different one.
Basically local public transport is a bit different from most other things, and a monopoly will often actually be good for the customer.