Why does Tuck say he and his family are like rocks beside the road in Tuck Everlasting?

Tuck says in the novel Tuck Everlasting that he and his family are like a rock beside the road. He means that they are sitting still, watching the rest of the world (and life) pass them by. With this quote, he is lamenting the true horror of immortality—one that is often overlooked. While the family has gained everlasting life, they have lost everything else. Their individual families have abandoned them and they can't embrace or experience the world like they want to.
The way they experience the world is shallow compared to other people; they act as outsiders looking in because they can't form lasting relationships or get deeply involved in a community without seriously jeopardizing their lives and privacy. In the end, while immortality seems like a wonderful fantasy, it is a horrible curse for the family who can't enjoy life like they want.