It's not in insult, it's informative. E.g., would a blind person ever realize they were missing one of their senses if no one told them? Consider it a service to some posters... maybe the first time they're called an idiot, they might dismiss it as a simple insult. But, after 90 or 100 times, you might expect that mayyyybe, there's something to it. I know that if a few respected posters said that my opinion about a particular point of view I was espousing was "stupid," I might reconsider it. E.g., "I don't believe in vaccines, natural immunity is better." 7 or 800 "you're an idiot"s later, you would think it would sink in.
Now, about not realizing this, OP. . .