From what college? And moving into then out of SF doesn't make the bay area a revolving door. My (bay area) college class was half from CA, half from the north

From what college? And moving into then out of SF doesn't make the bay area a revolving door. My (bay area) college class was half from CA, half from the northwest. About 50% of those from the northwest moved 'home' after school. Is that a revolving door? Did they get sick of the area? Or did they never intend to stay?