I don’t think the following is the whole story but there’s plenty of truth in your assertions nevertheless:
Most of the threads, if they run long enough or concern some subject near and dear to the hearts of the posters, end up being insult contests.
Some don’t like to be challenged in their beliefs and take each challenge as an attack, others can’t or won’t defend what they say because they haven’t ever really given their beliefs deep examination and think sarcastic repartee’ is the same thing as defence.
And others are just arrogant, too arrogant to ever admit their views might be mistaken no matter how much stands against their opinion in evidence of experience so they insult or ridicule as if that proves the other person wrong and themselves right.
Mix all that with a generous dose of self righteousness and the one upsmanship game never ends.
Forums like FR that provide a soapbox with some rules to restrain those who can't or won't restrain themselves and also a place where such high-minded and noble persons like ourselves (well...maybe a few others) can make spirited arguments and allow that even honest people can have false ideas.
So hopefully we'll all remember that if God can put with fools and knaves for such a long time surely we can for such a short time.