Upvoting based on "this comment brings up good points and arguments" rather than "this comment says things that I think are true" helps fight against the community becoming an echo chamber.

We downvote despite "good points and arguments".

The community is in many ways already an echo-chamber; that battle was lost a few years ago. Don't let it bug you.

Isn't "good" just as subjective as "things I think are true"?

The problem isn't subjectivity, it's groupthink. "Good but wrong" posts are better than "bad but correct".

Exactly. It's not that interesting to be in a forum where people are only awarded for being in line with the popular opinion. The alternative is to award constructive contributions.