The atheists I know LOVE to celebrate Christmas. They just make it a day to give presents, and think of Santa Claus, Reindeer, snowmen, trees, snow, Grinches, and any number of distractions, that have little to nothing to do with the birth of Jesus.
If you assume atheists hate Christmas, you’re assuming they are rational, logical, and consistent to their world-view.
They are not. They cannot be.
Most of my fellow atheists who celebrate Christmas simply consider it a mid-winter holiday that was previously pagan, co-opted by Christians, and represents winter solstice, or is just an excuse to brighten up the winter blues. Personally, I enjoy it. I don’t celebrate it myself anymore, but I like the pretty lights all over town.