Why did Angus Tuck say that they are "like rocks beside the road"?

The Tucks are "stuck", forever watching life go on around them, but never changing. They are like rocks beside a road in that they are unchanging, like rocks, and they are beside the road of life. They are not traveling down the path like everyone else, but they are beside it, not going anywhere, never aging or changing. Just like rocks beside a road, the Tucks can watch people's lives go by and the world change around them, but rocks never change, they just sit there. Angus Tuck uses some great metaphoric imagery in describing what it is like for the Tucks, who are frozen as they were when they drank from the spring, never growing old, never dying, never changing, while the world goes on around them in its natural cycle. It is a painful thing for the Tucks, who wish that they could have lived a normal life.