Yes, “in theory”. But in practice other people might be better at pretending than the person who has lived it. Maybe because of them being better actors, because of having more people to chose from, because they fit the director’s vision better, because of them being a “blank canvas”, etc.
I get the theory, but it does not necessarily translate to practice. Also, where would it stop? Only mothers should play mothers on screen? Orphans cannot play in movies with parents? Only musicians/scientists/etc. should play respective parts?
You don’t have to have lived it to play it. That’s why it is called acting.