Because it's impossible to cover everything in a completely balanced way.
Explanation:
A bias is a factually unfounded notion, that is, a preconceived assumption about someone or something, based on the application of a stereotypical notion of a group to which the person or person who is the subject of the prejudice is considered to belong. In the absence of information about someone or something, knowledge gaps can be filled with general stereotypes. A bias can, for example, be based on probability and empirical evidence instead of statistical factual knowledge.
