Surprisingly few have asked me this question, although it is one I have asked myself repeatedly, mostly in order to make sure I know the answer and that it is a good one. In fact, the answer is very simple and also very sound. New churches tend to reach new people. It is not that older churches cannot or do not reach new people, but new churches have been shown to be the best way to reach new people in many contexts around the world. Of course, not all new churches succeed in reaching their communities, and some do not even survive. However, enough survive and thrive that it continues to be worth the effort. Indeed, 100% of the churches that exist today were once new churches. I am thankful that someone made the effort to start them and am grateful to have the privilege of participating in the start of another one.
For a more thorough answer (but not too long), here is an excellent article by Tim Keller, who has planted churches and encouraged their planting around the world.