So, is non-confrontationally and constructively trying to add to a conversation one of those skills that some people can't master, one of those skills that some people aren't given the opportunity to master, or one of those skills that they just can't be bothered to try mastering?
All of the above... As far as the last one goes, I've known some incredibly well-spoken, rational people who are vastly entertaining conversationalists who make you feel smarter for having talked to them... As long as you don't hit their particular trigger subjects that turn them into slavering fanatical trolls with a torch in one hand and a noose in the other, completely incapable of anything except viciously attacking anyone that doesn't share their own viewpoint.
I have been spoiled that the vast majority of people I have run D&D for have been the "whatever you want" type and are down to play the world and those who haven't been that open to anything have still been pretty easy to compromise with - save a few notable exceptions that in retrospect we were never gonna find a common ground of expectations because of personality* or game style.**
* I had one player who was the D&D equivalent of that kid at Christmas who always preferred what another kid got over his own gift no matter what it was.
** I had one player who was a "build-first" type, and was kind of passive-aggressive when he didn't get an opportunity to use what it his character was built for all the time.
I can't speak for everyone, but I can say that most of the games I've played have started with folks who were relative strangers at the time.
While most of my interactions have been positive, there certainly have been some rough patches. It's almost never really about the game.
One thing I'd say, in my experience mostly with playing online, the power imbalance between the DMs and players is far more significant than what I've seen at a home game. Where there are communications failures or disagreements, the player has to worry about being kicked from the game with no real recourse. So, as a player, you sometimes wind up tolerating a measure of discourtesy or unpleasantness you would not tolerate from friends, because playing is better than not playing.
So I don't really find a level of distrust toward DMs that surprising.