In John 3:16 why did God say that he love world and not  just indicate mankind....?

The world is used as a metaphor for people.

Simple.  Metonymy of the subject is the figure of speech used here, as it readily suggests the inhabitants of the "world".

I would say it was a sort of code. Jesus is shown using World (kosmos) which sounds inclusive of everybody, but in John 14:17 "The world cannot accept him, because it neither sees him nor knows him. " It's the same word. There is nothing special about it that puts a negative connotation to it. That's why I say it's code. Or like a code. Jesus loves the world of believers. He does recognize that the world which is against him is not going to be able to accept him. Later in John 15 he says, :18 “If the world hates you, keep in mind that it hated me first." That world which isn't able to know him can hate us who are in his understanding.

For every living thing on this planet Earth has been given a measure of God's Holy Spirit and not just in man.