And all the prophets prophesied so, saying, Go up to Ramothgilead, and prosper: for the Lord shall deliver it into the hand of the king. (2 Chronicles 18:11)
The prophets prophesied as they did because this was the message that Ahab desired. They were in the service of the king of Israel, not in the service of the Lord of heaven. In telling Ahab to go up to Ramoth-gilead they were telling him to go to his death (see v. 34).