Why common sense knowledge about society is incomplete?

Because it is partial, ie is not complete and not objective. It is in everyday language, not in theoretical language. It is not taught in schools and universities. Not everyone has commonsense.

Because it is partial, ie is not complete and not objective.

Our social context shapes our opinions, beliefs and expectations about society and social relations. These beliefs are not necessarily wrong, though they can be. The problem is that they are ‘partial’. The word partial is being used here in two different senses – incomplete (the opposite of whole), and biased (the opposite of impartial). So our ‘unlearnt’ knowledge or common sense usually allows us to see only a part of social reality; moreover, it is liable to be tilted towards the viewpoints and interests of our own social group