Atheists: Why are countries with the highest standards of living the same countries that embrace Christianity?

Because the people in those countries followed human nature -- self-interest -- rather than Christ's admonitions against wealth aculation. People ultimately serve their own purposes and that is usually a good thing (except when it causes them to harm or disregard others). These countries that "embrace" Christianity generally treat Christ as a mascot -- they sing His praises and claim to worship Him, but they don't really incorporate too much of His message in their own lives when it isn't convenient to do so.