We have done alter calls in the past and we will probably do them in the future. I tend to use this only when I feel like the Holy Spirit is telling me to do it. I don't have a strong gifting as an evangelist, but I am more a pastor/teacher, so I only do them if I feel prompted.
I am guessing the question is, why don't we do them every week? The alter call is a tool to prompt people to respond to repentance or salvation following an emotional appeal by the minister after a salvation message. You do not find examples of this in the Bible (telling people to come forward to the altar) but rather you see a call to repent and be baptized for non believers. We were actually doing that at least once a quarter in the sanctuary after service until our inflatable hot tub sprung a leak.
There are a few reasons why we don't believe it to be a necessary tool for every service. We believe that the Church is for the believer. Unbelievers are welcome to attend, and many people get saved (We have been averaging between 30-50 baptisms a year) but my main job as a pastor is to equip the saints (believers) to do the work of the ministry. I try to share the gospel within my messages, but my main objective is to teach the Word of God so that believers will become effective Christians. Most of the people who get saved in our fellowship are a result of their healthy Christian friends who can lead them to Christ and pray with them to receive Christ. Their altar call is probably happening in a living room somewhere, and they have a friend to disciple them and hold them accountable.
Many Churches use the alter call at the end of service because every message they preach is an evangelistic message, (a call to salvation or repentance.) There will also be a strong call to the believer to bring their unsaved friends so that they can be saved next week. The error in this method is that the average believer is starved for the Word of God since they only hear salvation messages every week. They don't grow, and so they don't share their faith or do the work of the ministry.
The altar call was something that wasn't used until the early 1800's. It was popularized by an Arminian evangelist/revivalist by the name of Charles Finney in the 1830's. He used altar calls in his tent meetings, so many ministers began to use them in their Sunday morning services. Along with the belief that one could lose their salvation, the altar call gave a person the opportunity to be saved at least 52 times in a year. I personally think that is nonsense. Sadly I have been in denominations that believed that and did a weekly altar call. I can speak from experience that many of these Churches I attended, who used altar calls, counted the people who came to the altar, and recorded them as "decisions for Christ" to report to their denomination. If a minister could get a lot of people to the altar, he might be called to a bigger Church.
I can see the value of an altar call now and again, but I feel it is important to listen to the Holy Spirit and follow His lead. I have gone through seasons where I have called people forward for repentance, salvation or renewal on almost a weekly basis. These were fruitful times because I felt prompted to do so and God did the necessary work. On other occasions I planned it and called people forward trying to duplicate those special times, usually with little or awkward results.
Every Sunday we do have people in the front for prayer. I like this because if someone was feeling convicted they can come forward and counsel or get prayer. This also keeps it from becoming a performance.