Why Did God Have To Send Prophets To Preach About The Messiah? Why Couldn't He Just Send The Messiah?

Why weren't the leaders chosen by himself,Saul and David good enough to keep Israel free? Why couldn't God see Saul would turn against him? Or just keep his people free? He can send hail and fire on the egyptians and all the other plagues why not keep his people safe the same way? Adding on lightening, quakes like the ones he used to kill the partiers at Sinai when Moses came down from the mountain?????