Because God is God, and not man, and He wants the whole world to be saved. God offers the gift of eternal life freely to anyone and everyone. Some will accept it which others will not. That is precisely what is revealed in Scripture (John 3:16-21):
For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life. For God sent not his Son into the world to condemn the world; but that the world through him might be saved. He that believeth on him is not condemned: but he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God. And this is the condemnation, that light is come into the world, and men loved darkness rather than light, because their deeds were evil. For every one that doeth evil hateth the light, neither cometh to the light, lest his deeds should be reproved. But he that doeth truth cometh to the light, that his deeds may be made manifest, that they are wrought in God.
The problem I see with Calvinists -- and it is a very, very serious problem -- is that they lightly dismiss the words of God and of Christ because those words do not suit their doctrines of men. This is what the scribes and the Pharisees were guilty of. They elevated the doctrines of men above the Word of God.
That passage in John chapter 3 is actually a record of the words of the Saviour Himself. It is Christ who is telling us that anyone and everyone ("whosoever") may be saved. It is Christ who is telling us "that the WORLD through Him might be saved". And Calvinists oppose Christ and His words daily. That is an extremely serious matter. That is opposition to God and to the true Gospel.