I would assume that people do not know they are to tip. This is my first cruise, and although we did not book excursions I would have no clue that we were to do this. If I saw that people were not tipping, I would assume they did not know. Oftentimes, the people are in this situation for the first time ever. They may not know, or be prepared....they see others tipping, and then next time they will come prepared for that.
Of course there are others that have no intention of tipping, but we cannot assume who they are.