Why are so many sermons primarily evangelistic?
When the church gathers, the vast majority of the people are either saved or at least think they are saved. Those who are saved certainly do not need to hear primarily evangelistic sermons. Those who believe they are saved, but in fact are not, have already heard the gospel so many times in church that they are highly unlikely to respond to it in that setting. If they were ever to accept the gospel, it would probably be outside the church building.
There certainly is a place for evangelism inside and outside the gathering of the body. Whenever I preach, I try to take some time to explain the beauty of the gospel. However, it is not my primary focus when speaking to the church. Sometimes I just make a short reference to the gospel if the biblical text I am preaching from is not directly related to it.
In the bible, I don't see people coming to Christ when the church assembles. Rather, it appears to me that the church is supposed to take the gospel out to the world by living it out and proclaiming it verbally. Matthew 5:14-16 says, "You are the light of the world. A city set on a hill cannot be hidden. Nor do people light a lamp and put it under a basket, but on a stand, and it gives light to all in the house. In the same way, let your light shine before others, so that they may see your good works and give glory to your Father who is in heaven." (ESV)
Jesus, in the Sermon on the Mount, spoke mostly about living out the Christian life. He was speaking to people who certainly did not understand fully what he had come to earth to accomplish. In light of that, it seems foolish for preachers to proclaim evangelistic messages to people in the pews who already know and believe what Jesus accomplished.
I'm speaking mostly from a Southern Baptist (SBC) background because that is where I have spent the past 11 years. Most SBC churches are currently plateaued or declining in number. My personal experience is also that many SBC churches are fairly shallow in doctrine once you get past the gospel. Please do not misunderstand me. The gospel is critical. Without it, nothing else in the Christian life matters.
SBC churches, for the most part, do not need another sermon on the death, burial, and resurrection of Christ. The members of the body need, rather, sermons that focus on issues such as personal holiness, marriage and family, prayer, servanthood, and the biblical functioning of the church.
Our SBC churches need spiritual depth. I imagine most churches outside the SBC also need spiritual depth. One great way for this to take place is for the preacher (assuming it is one person because in most churches it usually is), to preach through books of the bible. He will then be forced to preach about topics that might not be his favorites. In this manner, he will cover a much broader range of issues. This will, in turn, strengthen the church.
Let's keep the gospel at the center of the church. Let's not keep the gospel the sole focus of the church.