I definitely don’t hold this view, not being a secular atheist/humanist, but I’ll give you their point of view:
They know better than you do about what private decisions you SHOULD be making for the “greater good”.
They believe they have enough information, intellect, and morality to make those decisions better than you can.
The “invisible hand” that produces “greater good” unintentionally when everyone acts in their own best interest is a MYTH. The “greater good” must be planned and implemented by those who are better equipped to do so.