Why are people so unwilling to cummunicate with anyone who doesn't understand something or agree with them? [I Want To Share My Feelings And Thoughts]

Like, if there's something I disagree with or I don't understand and I ask about it, very rarely do I get to have a conversation with them about it.
I'm just trying to communicate with people, maybe learn a thing or two, maybe they could learn something as well. And then there are things I've left unsaid which bothers me, especially if it would have made a difference. I just know from experience that longer replies are even less likely to get a response so I try and save some things for later. And sometimes I do understand and/or agree but I want to say something from another point of view as part of the discussion or to see how they'd feel about it.