You have to understand that moderate positions or centrist positions are compromised positions. That means that you start with a position you have, then someone takes up a different position and then you change your position based on the relative location of their position. That’s not a great way to go about arriving at positions. In fact, it’s a guaranteed way to never actually get anywhere because your opponents merely need to go more extremely in one direction and you’ll just get dragged along.
What you think of as extremists on the side of communism are people with positions that have literally been around for over a century and have been based in an adherence to scientifically analyzing human society to arrive at their positions. Does that make them extreme? Would you say the same thing about climate scientists? Do you think it’s extremist to hold firm to positions that have been well and thoroughly analyzed and arrived at through rigorous study and debate?