It’s not cheaper although that’s only looking as the marginal cost of the journey. Once you factor in all the costs of owning a car, in reality public transport is cheaper.
That depends completely on your usage.
To sit on my drive, the car costs me ~£370/month (that's the car, servicing, insurance).
My mileage costs ~£15 in electricity/month
I'm currently WFH, but if I had to commute by train, it would cost me £16 for a return, so that's £320+/month - that on its own almost covers the cost of the car + servicing.
I drive my other half to work a few times a week as well, depending on her shifts, otherwise she'd need to get a train and bus, so that's another ~£7, or £12 to cover the taxi when they stop running, so another ~£20-30/week, or ~£100/month.
So that's £420/month, that's the insurance + electricity more than covered as well.
We visit my mum in Carlisle a few times a year, that's ~£180 a go (and that's just for the train, never mind the taxi fare to the station, or the taxi/bus fare on the other side), let's average that out, say 3 trips over 12 months = £45/month
We go camping 2-3x a year during the summer, we'd have to rent a car as there's no way we'd get all our camping stuff on the train, and then taxi to a field in the middle of darkest Wales, so that's another £100 or so for a decent sized car, lets average that out again, 3x£100 = £25/month
So, without any ad-hoc weekend trips, shopping trips, taking the kids to various activities etc., we'd be looking at: £320 + £100 + £45 + £25 = £490/month to get rid of the car and rely solely on public transport, e.g. paying at least £100/month for the pleasure of losing that convenience and comfort.