I think it's a location based thing, combined with how individuals handle their presentation.

My town doesn't have much in the way of public transport. We're just too small. The one bus route with stops here is more of a county thing, and doesn't have any real rider volume. It just doesn't go enough places to do anything useful for most people, and the stops aren't realistically available to most of the town.

But, when you use it, people tend to be friendly and polite, so there's conversation going on. The folks that don't want to talk tend to signal it clearly and it's respected.

The city I lived in for a while had bus and now has light rail. People there tend to be less chatty, but you also run into more people that aren't good at reading when someone doesn't want to interact, so some of that ends up being very terse.

But I've visited other cities, and you really can't peg down a nation wide kind of transit culture. It is true that random conversations happen, though. It's not a myth, or some kind of trope, or something that used to be true but disappeared.