They aren’t. The system is inherently not one way or the other. What you see is a result of human actions. We understood the notion of an echo chamber long before we had electronics. That is all a comment section is, an echo chamber. The only question is what echoes get repeated enough to be dominant.
It is polite human nature which leaves the trolls and arsehats seemingly dominating an echo chamber online we don’t like because we “take the high road” and block or ignore, and certainly not respond.
Consider the old adage “if you don’t have something nice to say, keep quiet” and how that plays out in an online forum. If the “nice people” don’t post what is left? Yup, arsewads. That isn’t the comment system being “set up” to emphasize toxicity (which loosely translates to “stuff I don’t like” anyway), that is the natural result of people trying to be nice.