If you think power is control over people and therefore zero-sum, any accomplishment is automatically the result of some exploitation

If you think power is control over nature (and only marginally control over people), cooperation becomes possible

Marx shared the labor theory of value with classical economists, which presumes a single scale of value for everything. This makes exchanges more for less (exploitative) or equal for equal (pointless) rather than A for B where both parties might benefit. As the left evolved from economic Marxism to cultural, racial, sex (etc.) Marxism, failure to explain economic cooperation became failure to explain any sort of social cohesion as anything other than dictatorial hierarchy.