I think it's a smart move from them to keep religion out of Sims 4 because religion is a very controversial topic and even if the religion in the game wouldn't necessarily be a real religion it would probably have some influence from different religions which would end up upsetting a lot of people who have different beliefs. Everyone believes (or don't) in different things and the beliefs can be a very touchy subject for some people (some people can feel very strongly about their own beliefs and think other religions are wrong etc)
Obviously, if a modder wants to make a religion mod then there's no problem, I guess.