What might be a better question is, Why, when Jesus said to pray "not in vain repetitions as the Heathen do" have two millennia of Christians vainly repeated the words attributed to him, and which we call "The Lord's Prayer".
Just supposing for a moment that Jesus actually lived, and supposing even more that he was, as is claimed, a manifestation of God, isn't it more likely that he was really saying, "When you pray, don't turn your prayers into a mantra. Don't turn them into a wish list, either. Here's a model you can use but for Christ's sake don't just repeat it over and over without meaning."