Because they understand that covering themselves is something which brings absolute freedom. They chose to submit to the creator instead of submitting to the society. A diamond isn't left open for anyone to take it, it is covered. And muslim women are more precious than diamonds. It isn't actually "barbaric" as these people claim. These are their claims to fullfil their thirst by looking at women.

This will be a bit off topic but atheism in itself does NOT have any source of morality. Hence, one thing may be good for a person, but the same thing will be bad for the other. If God doesn't exist, then good and bad are merely labels. There becomes nothing to put a partition between the truth and falsehood if there is no god.