Why do some people use Mark 10:18 (or equivalent verses from other gospels) to argue that Jesus is not God?

Your argument is the one that is not good here.

Jesus is clearly saying he is not good, at least in the standard of God. And you expect me to believe that he's REALLY saying he is good, despite saying the complete opposite. Silly.

Jesus is God as believed by 2.4 billion for over 2000 years; thus He himself declared as the Good...

Jesus is not God, His Father, He is A God

Those who argue that Jesus is not God believe that Jesus is but a created creature. Those who claim that Jesus was God incarnate argue that the Bible shows He was never created, so that's the main area of debate. However, once you believe that Jesus was created by Jehovah God, you gladly jump on any verse that could be taken to support such a view such as this one. But consider how the man fell on his knees before Jesus - more than just a respectful approach to Jesus as a good teacher! Jesus did not deny His own goodness, but He was making the man recognise that his only hope was in total reliance on God, not his own acts, not on what he had to do. Likewise, Jesus was not denying that He was God incarnate, otherwise He would have said, "Why do you call me good? I am not God!" 

What a perfect opportunity for Jesus to declare that He was NOT God incarnate, if that had been the truth of the matter! Yet Jesus did not deny being God. Nor could any Christian deny that Jesus was not good. There simply is no verse in the Bible where Jesus denies being God incarnate. That is highly significant. 

For the rich man, he was being challenged to stop thinking in terms of what he could do to merit God's approval, but to give up that which was holding him back from being a follower of Jesus - his great wealth.  It's interesting that Jesus felt love for this sincere man. Who knows? Perhaps after Jesus' death and resurrection, this man became a Christian. 

I think the Son is referring to His Father as Good.

The Son submits to the will of the Father.

For I have not spoken from myself, but the Father who sent me has himself given me commandment what I should say and what I should speak;

John 12:49

I can do nothing by Myself; I judge only as I hear. And My judgment is just, because I do not seek My own will, but the will of Him who sent Me.

John 5:30

38For I have come down from heaven, not to do My own will, but to do the will of Him who sent Me.

John 6:38

"My teaching is not My own," Jesus replied. "It comes from Him who sent Me.

John 7:16

Can’t answer that question, but there are many verses that prove Jesus is THE SON OF GOD. Especially the last verse, where God acknowledged he was his son.

John 3:16 God Loved the world so much he gave his only begotten son, so that those exercising faith in him  may not die 

Matthew 16:16 You are the Christ, the son of the living God.

1 John 4:9 by this the love of god is revealed, that God sent his only begotten son...

Galatians 4:4 When the full limit of time arrived , God sent his son

Matthew 14:33 Then those in the boat obeyed him saying; you really are a gods son.

Acts 9:22 And immediately in the synagogues he began to preach about Jesus, that this one is the son of God.

1 John 4:15 Whoever it knowledges that Jesus is gods son, God remains in union with such one and he is in union with God.

John 3:36 the one who exercises faith in the son has everlasting life

Matthew 3:16 After being baptized, Jesus immediately came up from the water; and look! The heavens were opened up, and he saw gods spirit descending like a dove in coming up on him. Look also a voice from the heavens said; this is my son, the beloved, whom I have a proved.

Actually Jesus the Christ of Nazareth, the Messiah, the Word and the Light, is God incarnate as John 1:1-14, 14-9 and His name Emmanuel/Yeshua reveals but in Mark 10:18 Jesus, as an example humbles Himself for other to follow in His stead.

What belongs to God, belongs to everyone, and is his due.

The Ruler called Him good but did not recognise Who He was. Jesus put His finger upon that.

He never asserted Who He was but always confirmed to those to whom it had been revealed from the Father.

He never contradicted the requirement of revelation from the Father.

It is written in the prophets, And they shall be all taught of God. Every man therefore that hath heard, and hath learned of the Father, cometh unto me. 

I have no idea -- because it isn't necessary to argue at all. There is no argument.

The FACT is simple: imaginary creatures ARE NOT REAL and DO NOT EXIST.

Nothing you do, no cult book you read - NOTHING will ever change that fact.

Middle Earth isn't real, Leprechauns aren't real, Zeus isn't real, jesus isn't real, Voldemort isn't real, Poseidon isn't real, the millions of Hindi gods aren't real, Bilbo and other Hobbits aren't real, Santa Claus isn't real, neither is the Tooth Fairy...

Imaginary creatures ARE NOT REAL and DO NOT EXIST. Deal with it.