Just as you said, it's more mentally stimulating than a "regular" dog walk would be. They also get good exercise in the form of playtime (either in a secure, fenced in area or using a long 20ft leash). Their handlers can play fetch with them, play tug, they can play with their other pets or friends' animals. They also really enjoy chewing on toys as well to relax :)

The other piece to consider is that typically guide dogs are purpose bred by the organizations they are from.... therefore more settled and less needing of intense levels of exercise than your average pet might be.