Q: Why do atheists bother getting up in the morning?
A: Because we can't reach the &@#! alarm clock!
In a nutshell, what this question really implies is that the questioner believes that all meaning in life comes from God. If you don't believe in God, therefore, your life can have no meaning, so why bother?
Let's turn the question around, however, and see how it sounds then. "If as a Christian your whole goal in life is to die and get to Heaven, why wait? Just off yourself now and cut out all those pointless years of mortal existence." I can hear the spluttering now, but the original question is just as silly to an atheist as this one is to a Christian.
Atheists find meaning in all kinds of things. We have families to feed, bills to pay, causes to advance, friends to see, lessons to learn, achievements to attain, and mountains to climb, just like theists do. Giving meaning to something yourself, instead of having it imposed by a "higher power", doesn't make that meaning less real. Is the love someone feels for a spouse they themselves chose less powerful than the love someone feels for a spouse their parents chose for them in an arranged marriage? Does it matter that the feeling is self-assigned rather than other-assigned? As long as it's sincere, it doesn't seem to me that one is better than the other. Just different.