Because Jesus also said Matthew 5:29, immediately following Matthew 5:27-28.
I disagree, respectuflly, that Jesus "explained that the law was in and of itself not complete" and then you said "Jesus did not add to the law." That is an apparent contradiction. Perhaps I am not following in understanding what you are saying.
Did Jesus add to the law or did He fulfill the law?
Deu 12:32 What thing soever I command you, observe to do it: thou shalt not add thereto, nor diminish from it.
Deu 4:2 Ye shall not add unto the word which I command you, neither shall ye diminish ought from it, that ye may keep the commandments of the LORD your God which I command you.
Deu 13:18 When thou shalt hearken to the voice of the LORD thy God, to keep all his commandments which I command thee this day, to do that which is right in the eyes of the LORD thy God.
Jos 1:7 Only be thou strong and very courageous, that thou mayest observe to do according to all the law, which Moses my servant commanded thee: turn not from it to the right hand or to the left, that thou mayest prosper whithersoever thou goest.
Pro 30:5 Every word of God is pure: he is a shield unto them that put their trust in him.
Pro 30:6 Add thou not unto his words, lest he reprove thee, and thou be found a liar.
Did Jesus add to the law or did He fulfill the law?
Mat 5:17 Think not that I am come to destroy the law, or the prophets: I am not come to destroy, but to fulfil.
FULFIL = STRONG'S G4137
πληρόω
plēroō
play-ro'-o
From G4134; to make replete, that is, (literally) to cram (a net), level up (a hollow), or (figuratively) to furnish (or imbue, diffuse, influence), satisfy, execute (an office), finish (a period or task), verify (or coincide with a prediction), etc.: - accomplish, X after, (be) complete, end, expire, fill (up), fulfil, (be, make) full (come), fully preach, perfect, supply.
The following scripture also utilizes the word "Pleroo, Strong's G4137" :
Rom 15:19 Through mighty signs and wonders, by the power of the Spirit of God; so that from Jerusalem, and round about unto Illyricum, I have fully preached the gospel of Christ.
Jesus did not come to destroy or anull the law in any way. He came to fully preach as a Master/Rabbi that the law is still valid. Much of the heavy burden and yoke upon the people that the Pharisees and Saducees had enacted, was just plain out "over done" and Jesus came to take that burden off of them by fully preaching the law. As OT scriptures show, Jesus could not add or diminish from the law given to Moses. Jesus kept the law perfectly and His example of how He did so is for our understanding.
You cannot have Jesus adding to the law, as you have stated, so "pleroo" makes total sense.