Explaining the tunnel system in their PopSugar piece, the anonymous former Disney employee said, “Cast members shouldn’t be seen outside their area because it hinders the experience. It doesn’t make sense to see someone in a Frontierland costume walking across the park to Tomorrowland to meet up with a friend. Thus [sic], the tunnels.”
As previously explained, these tunnels are considered to be “backstage,” as they are hidden from public view. Likewise, cast members have their own dressing rooms and cafeterias to use so that they don’t have to change or eat in front of guests. After all, that might spoil the impression that Disney has worked so hard to create.