I can answer that: because I like all the pretty, glittering lights, and gifts, and big dinners with family and friends! And the Christmas tree smells really good.
All flippancy aside, many of us atheists are still what I call "cultural Christians." We think a lot of the Judeo-Christian morals and values are very positive. Unlike Islam.
Maybe then you are not an atheist. Maybe you're just Agnostic.