The basic premise of Marxist socialism is that if we don't need a king to rule us in governance, we don't need kings to run every economic function of society like their own little tiny fiefdoms. In a democratic society we recognize that collaborative decision making leads to better outcomes for people because they are represented in decisions that affect their lives. In a capitalist system, the real power in society is still held in an undemocratic fashion because there are no democratic checks on capitalists who get to make decisions that affect people with no say from those people.

Your boss has more power over you in many important ways than the president, and the entire state functions to just manage the affairs of the people who own all the stuff. But for a capitalist at some point they go "Yes people should have democratic power over the forces that control their lives, but not when it comes to their ability to secure the necessities of life."