On Dec. 1, for the first time since 1968, when the Trojans men hosted Nevada, there were no Song Girls at the Galen Center. And Saturday afternoon, when USC hosts crosstown rival UCLA for a game that will be broadcast nationally on CBS, they will still be absent from the arena, despite a persistent outcry by Song Girl alumni, athletic donors and parents of current Song Girls who can’t fathom why USC would take away opportunities for students to do something they love.