I don't understand why people think that the only alternative to theism is nihilism.
It's not so much this. There are all sorts of alternatives to theism. It's just that nihlism, or something very much like it, is the only alternative plausibly more intellectually coherent. A person who says, "there is no god and yet I ought to avoid murder and practice charity" is plainly talking nonsense. You can't even begin to get an ought, in the moral sense, out of nature. "You ought" do nothing. Or anything. Or everything. "You ought not" do everything. Or anything. Or nothing. I don't mean this in the Ecclesiastes, Pascalian "there's only eternal annihilation at the end of the road and therefore all is vanity" sense. I mean something far simpler. There's no reason for you to adopt any permanent moral code. You could, plausibly, given that annihilation awaits us, justify some situational decision-making which looks a bit like morality. A 10 commandments for the atheist might look something like this:
1. Don't murder people in broad daylight. Justification: You will certainly be caught, possibly executed (which, again, leads to irrecoverable annihilation) and if not executed incarcerated, a happening that profoundly curtails your ability to order your few minutes on earth.
2. Don't commit adultery in full view of your partner. Justification: You will certainly be caught and dumped. Given that, even in our increasingly lax world, sex is by no means a certainty, purposely cutting off the spigot, even for one of your 10 minutes on earth, is unwise.
Etc. As you can see, the code really boils down to getting caught. Every choice is weighed on personal utilitarian grounds- do the benefits of this action outweigh the costs? If yes, the action is reasonable (there's no sense talking about "good" or "right"). If not, the action is unreasonable. This assumes, of course, that you still live in a moral universe, in which the majority of inhabitants recognize some absolute standards (even if you do not). In a universe where everyone has abandoned absolute morality and adopted personal utilitarianism you don't even have these sort of useful commandments circumscribing your behavior.
The really tough thing though is that charity is perfectly useless. It has minimal use in a world where you're a renegade utilitarian- after all, some pathetically moral soul might reward you for being a good samaritan. In a world where everyone has seen the light, charity is idle. It's no good saying, "yes, but I might have been born mentally handicapped or blind or in a third world country" because, after all, you weren't born that way and since you're going to be annihilated any moment now it is by no means probable that you'll ever personally benefit from a regime which treats these people kindly. You'd be reasonable to take minimal steps to foster a social mores that insures against any likely events (for instance, you'd be wise to promote hospitals) but you have not the least incentive to go out of your way to help someone.
This is the way we ought, rationally, to suss out decisions in a godless world. Anything else is a useful fiction. There's something to be said for uselful fictions in godless universes but they're plainly no more intellectually satisfying than the professed non-fictions of theists in Godful universes. Less intellectually satisfying because one can't even make them sensible on their own terms- they're merely quite deliberate lies crafted to order a senseless universe- whereas theism has embedded in it the notion of a telos (a purpose). It would be perfectly useless to adopt Christianity as a useful fiction because the whole point of Christianity is that it is not a fiction- if it is a fiction, you have believed it in vain and possibly forfeitted much of the pleasure you could have received prior to annihilation. Much better, assuming you're adopting fictions, to adopt something that conforms exactly to your own instincts and predilections. You enjoy hunting? Well hunting is your purpose. You enjoy sex but find blood gruesome. Well, unfettered sex is perfectly moral and murder verbotten.
The point is, that everyone who is NOT a theist does this because no one really lives as a perfect utilitarian. They craft useful fictions, often knowing they're fictions, in an attempt to order a senseless universe. Only theists believe something which really is useless as a useful fiction- they believe that they must believe. They believe that belief can, in some sense, walk about and flex its muscles and lift real mountains. Theism is the only thing in this or any universe that can possibly be TRUE.