That's because the law isn't the point of the passage but that Moses spoke of the Redeemer as well did the prophets...
Deuteronomy 18:17-19 And the LORD said unto me, They have well spoken that which they have spoken.
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.
And it shall come to pass, that whosoever will not hearken unto my words which he shall speak in my name, I will require it of him.
Deu 18:18 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.
Deu 18:15 The LORD thy God will raise up unto thee a Prophet from the midst of thee, of thy brethren, like unto me; unto him ye shall hearken;
Deu 18:16 According to all that thou desiredst of the LORD thy God in Horeb in the day of the assembly, saying, Let me not hear again the voice of the LORD my God, neither let me see this great fire any more, that I die not.
Deu 18:17 And the LORD said unto me, They have well spoken that which they have spoken.
Deu 18:18 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.
Deu 18:19 And it shall come to pass, that whosoever will not hearken unto my words which he shall speak in my name, I will require it of him.
Deu 18:20 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.
Deu 18:21 And if thou say in thine heart, How shall we know the word which the LORD hath not spoken?
Deu 18:22 When a prophet speaketh in the name of the LORD, if the thing follow not, nor come to pass, that is the thing which the LORD hath not spoken, but the prophet hath spoken it presumptuously: thou shalt not be afraid of him.
Deu 12:31 Thou shalt not do so unto the LORD thy God: for every abomination to the LORD, which he hateth, have they done unto their gods; for even their sons and their daughters they have burnt in the fire to their gods.
Deu 12:32 What thing soever I command you, observe to do it: thou shalt not add thereto, nor diminish from it.
Now whom do you suppose was speaking in verse 32?