Because Prophecy Or prediction, was one of the functions of the prophet. It has been defined as a "miracle of knowledge, a declaration or description or representation of something future, beyond the power of human sagacity to foresee, discern, or conjecture.
The great prediction which runs like a golden thread through the whole contents of the Old Testament is that regarding the coming and work of the Messiah; and the great use of prophecy was to perpetuate faith in his coming, and to prepare the world for that event. But there are many subordinate and intermediate prophecies also which hold an important place in the great chain of events which illustrate the sovereignty and all-wise overruling providence of God.