Short answer: they're not sight reading.

Longer answer: In addition to playing the guitar, I play / played the violin, viola, and French horn for many years. I performed in orchestras, small ensembles, in the pit for several productions, and in marching and concert bands for many years.

In musical performance, the term "sight reading" usually is taken to mean "reading a piece of music for the first time."

When musicians have sheet music in front of them in a performance, they're not "sight reading." They have been through those pieces many, many times in rehearsals and on their own. They've essentially memorized those pieces by that point.

The sheet music is there for the same reason that a teleprompter might be there for someone giving a prepared speech before a large audience. It's there as a reminder, and as a way of keeping everything coordinated.

In fact, most musicians performing in an orchestra could probably play the majority of their parts by memory by the time they get to performance.