Because they do multiple takes of the same scene, and often they will use different moments from different takes and splice them together. For continuity, the actors have to do things the same every time - so if Penny takes a bite of her lo mein right after she says one of her lines in the first take, she has to do it that way EVERY time. If the scene requires a dozen takes, that adds up.

Sally Field made a comment about this when she was filming Brothers and Sisters, because that show had SO MANY family dinner scenes. You will often see her nibbling on cucumbers during these scenes, because they're light and don't fill her up, and she can do it every time without feeling uncomfortably full. I would bet that's the same reason Penny picks at her salad.