Because that, of course, is what this holiday is really all about. Not about the food (though that's great), not about the family (though celebrating together is fine, too), not about the football (I probably won't even watch it).
There are several Bible paragraphs dedicated to givingThanks, probably as many as to God's will. But there is one special sentence in a short letter that ties them together.
Be glad at all times, don't give up praying, give thanks for every thing; for this is God's will in Christ Jesus for you.
1 Thessalonians 5:16-18 (my trans.)
One sentence - simple, yet profound. At all times, for every thing, don't give up. Why? Because that is God's will for you. That's one of the reasons Jesus came - to create a people full of never-ending gratitude.