Atheism still has a stigma associated with it here. Not sure now, but there were surveys less than 10 years ago saying that Americans were less likely to vote for an atheist than for a convicted felon (and this was before Trump!). Other polls often use the term "Nones" to refer to people who don't affiliate with a major religion, but that tends to include atheists, agnostics, areligious people, and some others thrown in there.
There's a good explainer here: https://www.pewresearch.org/religion/2024/01/24/religious-nones-in-america-who-they-are-and-what-they-believe/
That poll has 17% of Nones as atheist, which would be about 5% of the population, just above the 3% in the chart from OP. Although I bet part of the "agnostics" are agnostic atheists, but don't use the term. That being said, Pew also has this page: https://www.pewresearch.org/religion/2025/02/26/age-race-education-and-other-demographic-traits-of-us-religious-groups/#age
That shows that people under 50 are way different. 73% atheist of 69% None is about 50%, which means that a full half of Americans under 50 identify as atheist. So, a BIG generational gap.
I loved this show! But the abrupt "ending" just left so many plot points unanswered that I can't rewatch it at all, because I get disappointed. The acting was quite impressive, especially Lena Hedley (Sarah Connor), who went on to play Cersei Lannister in Game of Thrones