If you are actually asking;

Because most people generally agree that a more equitable society is good, people being beholden to their employers in a relationship with an extremely lopsided power dynamic is bad etc.

But because if even filthy shitlibs agree with things like that there is no value to espousing them; you get no good-leftists points for a statement which costs you nothing to make.

But if you say the US deserved 9/11 due to it’s support for Israel, or that North Korea is actually a model state and all the reports of human rights violations are just propaganda etc. well you are clearly willing to burn all your political credibility with normies and so you must be an ardent communist and therefore worthy of respect.

In real life this effect is diminished because you are usually forced into interacting with people who sadly aren’t a fan totalitarian regimes or quite like the US and you see these people often enough that you maybe respect them and care about their opinions. So you naturally moderate your opinions so you can perceive yourself as someone they would respect in turn.

But if you live in an echo chamber, the statements which would otherwise carry a personal cost suddenly become free and still ingratiate you with you peers. And then it becomes a conception to see who can pick the position juuust within the echo chamber’s overton window but as far as possible outside the general public’s overton window.

See also: the young republicans mask off support for literal Nazism in their group chat.