Just about every culture has a winter celebration. Religious or not, a large portion of the population is struggling in the cold. The party around the solstice gives a distinct turning point in the year as the days get longer. So why not take advantage of the existing merch, the existing day off work, and the spirit of your friends and family and partake? Going with the flow and pretending to celebrate a holiday in a religion you think is fake anyway is easier than having the same conversation over and over with family. And don’t underestimate the power of marketing, consumerism, and the lingering imperialism. I’ve been to India and many people expressed their wish to visit New York City at Christmas. It permeates local culture, it broadcasts globally.

As others have said, you’re showing a small world view and admitting your sample is small. Christianity creates atheists that celebrate Christmas. Judaism, hannukah. Modern non-religious cultural holidays, the new year. Traditional religions, some combo of the solstice and moon phase. Even those of other faiths that live in areas dominated by Christianity celebrate Christmas, heathens and pagans alike.