I think they presume that we have only one life here on earth, and can have more fulfillment in that short life without the restrictions established by the commandments of an imagined deity. I think some of them are genuinely trying to help others to live more deeply fulfilled lives which they think is hindered by "religion."
No doubt they do think this. How ironic.
If there is no God, and nothing after death, then there can be no such thing as living a "meaningful" life. Our lives, in that case are no more meaningful than is that of a lizard. It's only purpose is to survive and reproduce. Why reproduce? To create more lizards who have no higher purpose than to do the same. Of course, a lizard may be said to serve a purpose, in that it keeps down insect populations, and it provides food for large birds, and amusement for housecoats. However, none of these creatures have any higher reasons for existence than has the lizard itself. Nothing means anything.
A human, unlike an animal, can work to improve living conditions for other humans (that is, to be a humanitarian), or to preserve the earth's ecology (that is, to be a conservationist), etc. But none of these things has any lasting meaning, either. The people whose lives are thus improved have no transcendent purpose for their own brief existences, and all life on earth itself will eventually be destroyed, despite the labors of conservationists, when the sun eventually burns out.
A million generations of humans may come and go before that point is reached, but all of them will be equally meaningless. Only if there is a transcendent plan (and planner), and an eternal outcome, can anything we do or enjoy be regarded as more than transitory, lacking any meaning, other than what we arbitrarily choose to imagine its meaning to be.
What a difference it makes to know God! I cannot imagine any other life that can give purpose, meaning, and the potential for personal fulfillment. I often wonder what the atheist believes that he has, as a benefit of his atheism, which can even begin to equal what I have.