Other people seem much more willing to wander up to someone I'm in the middle of talking to, say hi to them and start their own conversation with them, often about something I don't even know about

Frankly, this sounds grossly rude to me. Listening quietly until I understand the topic of conversation is exactly what I do. There's normally someone talking about something that interests me, though.

Getting a word in edgeways naturally can be quite an art, unfortunately, too.