I'm not 100% sure if I understand what you're talking about, but I think what you're trying to say is why historians don't follow the 'great man theory'
/u/lordhussypants talks a bit about the cons of looking at history this way
/u/hillsonghoods also has an great answer but in the context of the history of science
https://www.reddit.com/r/AskHistorians/comments/4cewq8/on_adolf_hitler_great_man_theory_and_asking/
Could you clarify what you mean here? Because biography is a legitimate part of history writing.
Biography works well for history in two ways. Firstly, it can provide a historical overview of an important person's life. This isn't the same as great man history, where someone is promoted as an exceptional being who shaped history on their own. A biography of Barack Obama for example, would recognise the pivotal role he played in American politics as the first black man to be elected President, and the role he played globally as the leader of a powerful nation. But it would also address the way his society created him, the way his path was influenced by the institutions he was working within, and the people who were around him as he played his role. Great man history is the sole focus on one person, whereas good history sees that people don't operate in vacuums.
The second way biography works well in history is to view a historical situation or period through the context of one individual. It needs to be done well, but when it is, it's very effective in tracing an event and its effects on the people not directly involved by looking at the everyday experiences of someone adjacent to the event.