This question is from a student at Columbia International University www.ciu.edu.
1) Our world is changing and that includes how people respond and become disciples of Jesus. Once churches are over 20 years old many begin to lose their flexibility to make necessary changes. They have diminished desire to be risk takers — often a necessary ingredient to be able to reach people.
2) Leadership tends to be expressed in a generation. As a new generation emerges the leadership for that generation needs ways to carry out the leadership mantle.
3) There is a false assumption in the question. Resources to reach the least reached and unreached on the planet is not exhausted by planting new churches where the church is already established. The problem is a lack of focus, commitment and generosity by the established church to reach those outside their circles of influence.
4) Often new churches are more invested in local, regional and global witness than older churches.