Have they ever communicated to you the basis of their meaning and purpose? I am asking this question sincerely.
Don't you see that if we are merely products of evolution, then emotions (love, hate, envy, etc) as well as our efforts to improve our condition and our chance for survival are merely evolutionary processes and nothing more. To assign any meaning to loving someone or striving to invent a vaccine or even working to develop a food source to feed millions is nothing but folly. At that point, we are no different than that lion on the African plain who acts as if it actually makes a difference whether it procreates or not.
You say atheism does not equal nihilism. How can it not? Further, I would make the claim that any atheist who claims to have meaning and purpose in his or her life would, upon self-inspection, find an inconsistancy in his or her worldview.
For an atheist(more rather an agnostic), the very belief that his life is a result of the cumulative effects of chance happenings through antiquity, that he is extremely lucky to be a being, is reason enough to seek betterment of his life, and those of whom he cares of, so that he may enjoy for a longer while, what luck has befallen him.
To not know of what happens after his death, is reason enough to enjoy life when he is alive.