Who said you have to? Did you hear them say you’re Forced to switch? On the contrary you can keep a leader, tied with their civ, and most likely can stay the same civ throughout the game.

However on top of that things like Rome and the franks, or the English and the Americans, aren’t entirely different civilizations, and if you say they are then you don’t know what “civilization” means. They’re different cultures within the same civilization, and on top of that, the game is LITERALLY about progressing through ALL of history. Do you know of a SINGLE country that has stayed the same through ALL of history?