Because deep deep deep down inside even the most fervent religous fanatics knows it makes sense.
Atheism absolutely makes sense! And so does nihilism.
This is one of the points of Ecclesiastes. Life has no power and no ability to give meaning to itself, from the vantage point "
under the sun." The last part being the critical point.
Atheism is a proclamation of one's inability to see above the sun and nihilism (vanity of vanity) is the logical conclusion for the thinking atheist. An Atheist that ends anywhere but in Nihilism has stopped thinking logically and created his own fanciful world and morals to live within and thus he falls into the "trappings" he derides others for.
It is only in a vantage point given from "above the sun" that causes life to be something more. Without this heavenly vantage point man cannot determine any sense to the world. Ecclesiastes puts it this way, that God has put eternity in man's heart except that no man may find it. Always searching and never finding, there is but one choice remaining (from under the sun) according to Ecclesiastes, to eat, drink, and enjoy oneself, though even pleasure is fleeting and meaningless.
I don't recommend the movie, The Weatherman with Nicolas Cage, however the philosophy of the movie ends exactly here! There is no sense about life, except to glory in one's self. This is where under the sun searching ends.
For the Godly the author concludes differently, that only the heavenly vantage is worth guiding one's life. In the context of the book his message is very much in line with Paul's confession that whether full or hungry, abased or honored, abounding or in need, he is able to be content because-of and through THE heavenly message - Christ Jesus. Here there is purpose and not a grasping after the wind.
All that being said, atheists should not be offended by this message, there is no cause; for if they are right and that Christ has not come and been raised from the dead, Paul also says that we should be pitied above all others.
So we, in the foolishness of the Gospel (under the sun), pray for the atheist that the work of the Spirit may reveal eternity in the truth of Christ Jesus, transforming them through the renewing of their minds toward God.
Respectfully Submitted