The answer to the "why isn't there an election" question that comes up whenever a Prime Minister resigns is that a Prime Minister is someone who can command the confidence of Parliament. Generally speaking a President is someone who is directly (or somewhat indirectly in the case of the United States) elected by the people. Aside from having the Head of Government title, Nancy Pelosi is much closer to being a Prime Minister than Joe Biden is, and there would be no demand for a national election if she resigned in the middle of a term.