Actually, my explanation is in perfect, flawless harmony with the facts.
Most of the stuff on the back of a dollar bill is completely meaningless to most people. The pyramid had religious significance when it was built, but who contemplates that? It definately has no religious siginificance now, and did not have any when it was put on the dollar.
Same thing with the phrase "In God we Trust" - as I said before, "God" is not the significant part of the phrase, it is "Trust." It could be "Peanut Butter" and the phrase would have more meaning for people.
Why is it not removed? I suggest a few reasons: legislative laziness (the phrase is meaningless, so why go through the effort to remove it?), the political threat of Christian evangelical nutcases (who, as we know, can be mobilized over stupid things), and tradition.
So the primary reason it's still on the dollar is that it's removal is not a significant enough issue for legislators --- and the Supreme Court has yet to find a legal reason to do so, again because it's an insignificant issue.