I don't know of any that opted out because of Covid. USC only had a couple of players opt out and they did so because they would have declared for the NFL after the season. This final season would have been their preparation for the NFL Draft. They were told that they no longer had that option or that, at best, they might have a 6 game season in the spring (and most were very skeptical about that happening).
Ya know what else is in the spring? The NFL combine and draft. So they decided they may as well just opt out and prepare for the NFL combine rather than be playing a meaningless 6 game season in college and have to miss the combine.
Now, the very people who told them that there would be no season, have changed their minds. So they have the opportunity to prepare for the combine in the way they had intended to before the lockdown.
Like I said, under normal circumstances, I would be 100% in agreement with you on this. But these are not normal circumstances.
This isn't one of those cases where some kid declared for the draft, then found out he wasn't going to get drafted as high as he thought and wants to come back. These kids were told they weren't going to have a season.
And ftr, I'm not speaking just about USC players. To my knowledge, USC hasn't had a player say they want to return.