Because it recognizes a god that some people do not believe exists?
"For example, both Houses passed resolutions in 1789 asking President George Washington to issue a Thanksgiving Day Proclamation to “recommend to the people of the United States a day of public thanksgiving and prayer, to be observed by acknowledging, with grateful hearts, the many and signal favors of Almighty God."" See Van Orden v. Perry, 545 U.S. 677 (2005).
You have clearly misinterpreted the intent of the first ammendment .