Prophet Hud - 11:12
"Perhaps your chest will be constricted and you will abandon some of what has been revealed to you because they say, "Should not a treasure have descended to him, or an angel with him?"You are only a warner; Allah is the Controller of all things."
Why a human messenger?
The Night Journey/17:95
"Say: "If there were angels inhabiting the earth, We would have sent to them an angel from heaven as a messenger."