I, too, love to generalize the reigning ideology, an artifact of an extremely recently developed system that has existed for only an incredibly brief period in human history as if it were something intrinsic to nature

Human nature arguments boil down to the fact that the ruling ideology of society is shaped by the base economic relations. In other words, we perceive it as "natural" to work for monetary incentives because that's pretty much the only choice we have right now. It seems "natural" that there should be hierarchy because we have them at our jobs, in our democracy, in the home thanks to patriarchal family relations, etc. Humans seem greedy "by nature" because without money, we starve or can't pay for shelter, so it is in our interest to try to accumulate money in under the current economical setup. Capitalism literally compels capitalists to behave in a way that can be perceived as greedy, because if they don't, their competitor will and then they will be gobbled up or put out of business. It's the system that makes these things seem natural or essential.

To attribute contemporary human behavior to "human nature" without looking at history and without considering the effects of the environment and social relations that necessarily shape human behavior is simplistic and unscientific, and it is usually an excuse made by the politically faint of heart or those who benefit from the current economic system.