It's not rocket science: being human is not in itself a character flaw. One of the fallacies of rationalism is in assuming that people should be expected to behave in all ways rationally. Reason is a circumstancial process, not a replacement for emotional and biological needs. Different circumstances have different priorities, and the priorities of the poor aren't the same as the priorities of the wealthy.
When you don't have many resources -- you're not getting the food or rest you need, you're constantly stressed, and you see few opportunities to relieve yourself of these problems -- all your time is consumed either getting by on a basic level, exhausted from the work it takes just to keep a roof over your head, or worring how you're going to make ends meet. Immediate practical concerns, including emotional needs, trump long-term planning even when there are resources to conserve.
Someone with a steely constitution, nothing going on in his life, and boundless self confidence can perhaps plow through the inertia of circumstance -- but as the saying goes, most people work to live, not live to work. Those few moments of respite, you cherish them. Because you don't know when you're going to get another. If things get better, great. One more monkey off the back.
Though I get that the original post is playing devil's advocate, the question is naive to the point of absurdity. It's the application of an abstract standard to a concrete situation. You might as well ask "Why aren't people always nice to each other?" Well, nice thought. There are plenty of reasons. Here, come live with me and I'll show you a few.