Because he still has to live in it - and can take a bullet to the head from the Morlocks just as easily as any altruist can. It is in the selfish person's rational self-interest to help build a society that avoids creating Morlocks, even if a few would-be Eloi are temporarily inconvenienced. :)
“Because he still has to live in it”
Tyrants and thieves can and often do thrive. The world is big and can stand a lot of ruining without negatively affecting them during their lifetimes. Mugabe, Castro, Kim Jong-Il, et al seem to be doing quite well for their selfish selves, thank you.
Really, Rand’s worldview cannot be justified without an unexamined and unexplained caring about the fate of society. That, and her system of morality which she seems to claim is simultaneously axiomatic and based purely on reason, makes her trashing of religion rather unreasonable.
The best basis and justification for individualism is Biblical. Without it, it’s a house without foundation, easily collapsed.