Why do some people say that in order for a man to preach he has to be called by God?
"Called" is another way to say "invited" or "encouraged to come."
Those who are Christians have been called by God. "You are witnesses, and so is God, how devoutly and uprightly and blamelessly we behaved toward you believers; just as you know how we were exhorting and encouraging and imploring each one of you as a father would his own children, so that you would walk in a manner worthy of the God who calls you into His own kingdom and glory" (I Thessalonians 2:10-12). This passage tells us who does the calling: God. It also tells us what we are called to: His kingdom and glory. But it doesn't directly state how that calling takes place.
Unfortunately, people have a habit of filling in missing information with imaginative answers. But we don't have to guess. We merely have to look in other places for the answer. In this case, the answer is in the verses that follow. "But we should always give thanks to God for you, brethren beloved by the Lord, because God has chosen you from the beginning for salvation through sanctification by the Spirit and faith in the truth. It was for this He called you through our gospel, that you may gain the glory of our Lord Jesus Christ. So then, brethren, stand firm and hold to the traditions which you were taught, whether by word of mouth or by letter from us" (II Thessalonians 2:13-15). It is through the teachings in the Bible that we are called by God.
Now, if Christians are called (invited) by God to partake of the kingdom, why would we conclude that men who preach are called (invited) in some mysterious way? Jesus said there is a need for people to teach the gospel (Luke 10:2). Men are trained for the duty (II Timothy 2:2).
See Preachers for more information.