Because individualism goes against our biology, against human nature, and against the literal definition of society. Individualism is not natural for humans. Society cannot exist in a world where it's everyone for themselves. Humanity did not get to the good places we are by individualism; in fact I would argue that all of the bad parts of society sre because of individualism.

Being against individualism doesn't mean we all have to be the same person, it just means that we acknowledge that other people exist with other perspectives and wants and needs, and we cooperate with them and they cooperate with us.