Is there any point in just saying hi to a person but never talking (meaning having a conversation)?

Like you said, sometimes it's just out of politeness. It's the same as when you go to a place, like the store, and you see a complete stranger that waves at you and you wave back. You're just trying to be nice because that tells the other person you still care that they are even there! Also, if it bothers you, you should make the next move and try creating conversations and try rebuilding those friendships, if you want to. If you really don't care I think you should continue saying hi to them because it lets them know that you know they are still there.