The system is highly organized
Class society is constantly becoming ever more highly organized, increasingly complex and globally interconnected. It has constructed solid institutions with which to exert power in every realm of life. The state is just one form, with its major power tools the military and police, along with its courts, legislatures, down to the smallest government office in the smallest town. Economic power is another tool of domination — the ruling class has seized control of the ways we fulfill our basic needs (starting with land, followed by all our means of subsistence/production). Our survival is now in their hands — they can threaten to withhold these necessities unless we agree to participate in our own exploitation through labor.
Additionally, power is asserted through ideological domination, via the culture: the media, the arts, customs and traditions, religion and so on, which persuade us (with varying degrees of subtlety) that this is the best of all possible social arrangements. Oppressive social structures such as the family, patriarchy, institutional racism and nationalism hinder our ability to unite and thus strengthens their position against us.