Cause Christians generally wont accept that they are anything less than universal, transcending specific cultures, and atheists of Christian background generally prefer to deny that THEIR particular viewpoints and biases are rooted in Christian civilization, rather than representing a universal truth
The other thing is that Christians define themselves by their creed not their background. You can have christians of every nationality and culture. It's a different relationship then Judaism. Christians aren't united by blood but by creed.

Remember, average life expencency does not tell how long most people lived, it also reflects all those who died as children, and those were many at the time.
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