People who are good at playing a technically hard piece essentially know it by heart. They couldn't play it on sight, they have invested significant time to be able to play it. It's just the few remaining percent of reminders and the comfort of having the sheet "just in" case as well as the being-used-to-it factor that makes them stare at the sheet. But if they can play it really well then they don't _actually_ need it. Any pro musician will tell you this.

The orchestra setting has the extra requitement that the sheet is a tool for communication. "The figure in bar 83" is not a term you have gained an intuitive understanding for, but is needed to communicate in an orchestra setting. The soloist though often times plays by heart, at least during performances, so as not to get distracted / get tunnel vision.