Most Lemmy users probably came from Reddit, and this is what they (or we) brought back from there.

The karma system encourages conformity by promoting popular opinios and hiding unpopular ones. People who see a comment with -2 karma will most likely have a bias against it as it is seen as something bad by the community, and this will lead to more downvotes.

Power mods who heavily curate and delete comments and posts also add to this.

Of course, this is not exclusive to Reddit or Lemmy, every community has their own set of acceptable opinions and you’ll most likely get challenged if you disagree. But the way Reddit and Lemmy are built makes the creation of echo chambers much easier, that’s why you see the same opinions from most users.