There are atheist Jews in Israel. Is Israel part of the "cultural Christian west"?
So you need to explicitly define atheist Jews as the exception.
I don't see how the identity of atheist Jews in Israel depends on non-Jewish people celebrating Christian holidays in the US (whether those people identify as atheists or not)
Then you have people that may have grown up surrounded by some of the trappings of Christianity, but because of their atheism, do not attend church and take part in only the most universal rituals (Christmas, basically, and even then, the Christian bits are mainly removed). Christian thought is not the moral or philosophical touchstone.
Until a few years ago religion was listed on government identification cards. When you’re born your parenrs must state a religion for your birth certificate or it is automatically listed as Muslim.
You had to go through a burdensome official process as an adult to change this, and once you did this you legally admitted to Apostasy, which opens you up to discrimination (and future consequences if the government were to fall to islamists or neo-ottomans).
Furthermore, it was very common to be discriminated against by HR departments / hiring managers if Islam was not present on your Kimlik (government ID).
This only went away with recent passport and national id standards changing in their futile attempts to join the EU.
(Consider that Turkey is by far the most liberal and secular Muslim nation.)
How or why they started or what the religious think of them now is irrelevant.
Rename lupercalia to Valentine's Day, Saturnalia to Christmas, etc. To make Christianity a seamless switch for the Roman populace
I think there are more of you floating in the pews with you than you may expect. As I've grown older it's been hard to belong to a dogmatic religion. But I miss those same things you describe. I too attend on occasion for those exact reasons.
It was startling to me, however, to realize that I'm not alone. A large % of people there in the church find value even if they don't actually believe. And that's a strange comfort that let's me continue to appreciate the tradition without claiming the title.