Lefty vs Lefty Splits

[Wilson A2000]

When you are a lefty switch hitter and a lefty is brought into the game from the pen to face you, why does it automatically turn hitters around? Hitters who are much better hitters as lefties are forced by their own doing to hit right handed. Is is true that lefty hitters who might be better lefties than righties would be better off facing a lefty pitcher as lefty than as righty. Why do so many switch hitters get automatically turned around and lose their power and average?