I have anterior shoulder instability and my surgeon wants to open up my shoulder joint to repair it.  Is this necessary? Why can’t he do this less invasively with arthroscopy?

Surgeons make surgical technique choices based on multiple factors. Certain types of surgical repairs have better outcomes with an open technique, like your surgeon wants to do.  Also taken into account is the surgeon’s experience level with the type of repair and his or her comfort level regarding open or arthroscopic technique.  In your case, the “gold standard” treatment for instability correction is through open technique.