Which to me raises the more current question of why people have more children now than can be sustained...
The reasons I have come across so far are
a) they don't think about it,
b) they care little about delayed consequences,
c) they believe the planet can easily support 50 billion people as it is,
d) they trust that technological progress will provide,
e) they don't see why they should restrict their reproduction for as long as there is anyone else who doesn't.
Will human nature change that much because the "ship" is so much smaller than the planet-sized vessel we are on today?
No, but of the above reasons for not dealing with the problem, a) to d) are excluded by people already being hungry and nobody doing anything to increase food production. Even e) may not apply. We are not told how large the population is. Reference to camps suggests tens to hundreds of thousands, but the story is written as if everybody knows everybody else, which would mean up to 150. Even if it's 5000, they should be able to come close enough to a consensus that I expect a social norm on limiting reproduction would be pretty rigidly enforced. After all, they do enforce norms on how much anyone may eat.