Whats wrong with all those western countries mixing religion and government together?

Okay so it’s pretty logical and obvious that religion (including belief in god/gods) should be separated from politics and government and any governmental or political slogans and the idea of separation of religion and state is very logical. Now many western countries like in western Europe and the U.S still think they live in the dark Christian ages and medieval ages and mix their religious beliefs with politics for example the U.S pledge of allegiance says" one nation under God", British anthem says: God save the queen" and more and more silly is that Germany which is a western country as well is ruled by religious party called CDU: Christian Democratic Union and one of its ideology is Christian conservatism and they even collect mandatory church taxes that go to Christian churches which sounds very stupid to me because there are other religions and atheists as well who are not attending those specific churches. I mean what exactly is wrong with those western countries? Do they want another one of their Christian dark ages to happen again so they can send the world back to those stone ages when they had all those stupid Christian crusades killing people who are not Christians or what?