I guess that it is the religious freedom and religious diversity that keeps religions in USA more "alive". Religions compete with each other, try to convert each other's followers, keep the religious memes virulent. People work harder to signal belonging to their religion. And atheists living in the same culture work harder to signal their atheism.

In Europe historically you often had one mandatory religion per country. Without competition, priests got lazy and religion got boring. Some people lost their faith in religion, some people still believed in the religion but also saw the lazyness and corruption of the church, so you got many people who publicly identified as religious, but tried to do as little as possible about it. Those who identify as atheists are quite similar in behavior to the most lazy of those who identify as religious.

Metaphorically, the American religious landscape is a few separated shining colorful dots, atheism being one of them; the traditional European religious landscape is a gray bell curve, atheism being one of the ends. (The people you describe as "culturally still identifying as a Christian" are somewhere at 90% of the curve. There are also real atheists at the end, but they are fewer.)

I imagine that the increase of Islam might change this picture in the future; that the group of "lukewarm Christians" may decrease because (1) there will be attempts to convert them to Islam, and in response to that (2) the Christians may also wake up and start radicalizing their lazier members, and (3) some people will start identifying as atheists because they will dislike both of these options. Then, instead of the bell curve, we will have three separate groups.