They don’t believe in the God of the Bible or any sort of Intelligent Designer. If everything came into being through chance, there is no reason to thank anyone or anything.
So why do they celebrate Thanksgiving? Probably just tradition. Thanksgiving is a great holiday: time off, visits from family (could be a good or bad thing), feast on turkey. I guess one can use any excuse to produce a tryptophan-induced snooze.
Whether they know it or not, even atheists depend on God’s provision. The Bible says that God “. . . causes his sun to rise on the evil and the good, and sends rain on the righteous and the unrighteous.“ Matthew 5:45.
Even when people won’t admit His existence, still God is merciful.
Abraham Lincoln recognized this fact, even in the midst of the Civil War, when he proclaimed a Day of Thanksgiving in 1863:
The year that is drawing towards its close, has been filled with the blessings of fruitful fields and healthful skies. To these bounties, which are so constantly enjoyed that we are prone to forget the source from which they come, others have been added, which are of so extraordinary a nature, that they cannot fail to penetrate and soften even the heart which is habitually insensible to the ever watchful providence of Almighty God. . . . No human counsel hath devised nor hath any mortal hand worked out these great things. They are the gracious gifts of the Most High God, who, while dealing with us in anger for our sins, hath nevertheless remembered mercy. It has seemed to me fit and proper that they should be solemnly, reverently and gratefully acknowledged as with one heart and one voice by the whole American People. I do therefore invite my fellow citizens . . . to set apart and observe the last Thursday of November next, as a day of Thanksgiving and Praise to our beneficent Father who dwelleth in the Heavens.
Thanksgiving is not an excuse to overindulge. Rather it is an opportunity to recognize the bountiful blessings we have received from God, even though undeserved.
What do you need to thank God for?