Isn't god supposed to be powerful?

Deuteronomy 18:18 says, “I will raise them up a Prophet from among their brethren, like unto thee, and will put my words in his mouth; and he shall speak unto them all that I shall command him.” Then in 18:120 it says “But the prophet, which shall presume to speak a word in my name, which I have not commanded him to speak, or that shall speak in the name of other gods, even that prophet shall die.” Why would he raise a prophet and not be sure he would only say what he wants him to say? Can’t he control this person? Doesn’t he know everything? And if so why doesn’t he know if this person will say something wrong?