I don't think they really are, but there's lots of lonely people out there open to chance encounters. Transit's one of those rare spaces where diverse groups of humans gather, and chances are attend on a frequent schedule. Most folks on transit would prefer you just leave them alone, but hey, you never know. I've met people/had good conversations on busses before.

A thought: if you see one of these regulars again, try giving a nice compliment (with strangers, nice = one that doesn't involve their body - think a cool book they're carrying, t-shirt, dangly thing on a backpack, cool shoes, etc.) and a question (open ended if you feel like trying for a conversation, closed is fine if that seems daunting). Sometimes cool shit can follow - worst thing that generally happens (so long as you follow the advice afterwards) is you're told to fuck off. A lot of the time, it's something in a middle - the question gets answered, you might say "Awesome! Well I hope you have a great day" with a smile and go sit, and the stranger moves on with (maybe) a little boost. That ain't nothing - lots of us could use those little boosts. All of of a sudden you might see yourself mentioned in those missed connection posts, lol.

Despite having my own hot-and-cold periods over it, I truly feel we need to talk to strangers in meatspace as a whole more often.