That's by design. The Founding Fathers knew the dangers of a state church and took great pains to leave any reference to a national faith out of the new nation's founding documents. Personally, I've seen how well government does everything else and am glad it didn't try to handle God, too.
Despite all this, we do have a national religion. It's naturalistic evolution. Its proponents would argue that it's science, rather than religion, but it's an unprovable assumption about God, which sounds like a religion to me.
That's right, it's unproven and unprovable. You can't recreate naturalistic evolution under laboratory conditions, and you can't observe it in nature, so you can't prove it by scientific means. That's the reason scientists give for discounting God, so it is only fair to use the same criticism.
The problem isn't that it's all unproven hypothesis, but that it's treated as a fact. It's been said to be sure by the late paleontologist Steven Jay Gould, possibly the greatest popularizer of science since Isaac Asimov
Whenever a quote is needed from a "scientist", an evolutionist is chosen, usually because they're irreligious and thus unbiased. The only thing wrong with that is that no one is unbiased. Scientists are people, not robots, and they have bones to pick like everyone else.
Naturalistic evolution is an untested, untestable hypothesis. It is, in fact, a religion,one whose chief virtue is that it protects its adherents from having to believe in any God at all. Until they die.