I remember the late-night college conversations, those in which someone would make a claim about the basics of all human behavior and then back it up by pointing out that their claim must be true because it was a view espoused by Friedrich Nietzsche, or Adam Smith, or Karl Marx, or Immanuel Kant, or...well, you get the point. I had thought that this was the sort of thing that people grew out of shortly after they graduated from college, but then I shared an apartment with a friend who would frequently make statements about the state of humanity and felt that his positions were unassailable because they were espoused by Thomas Hobbes or John Locke (the philosopher, not the guy from Lost); I had family members become heavily influenced by Libertarianism, and cite Adam Smith's arguments in favor of capitalism as if they were unquestionably proven fact; and I lived in Santa Cruz, so of course I knew more than a few folks who were utterly convinced that anything that Karl Marx wrote was a revelation from on high*.
What was so strange to me about this is that all of these philosophers were laying out what amounted to untested or semi-tested hypotheses about human behavior. Yes, the philosophers themselves didn't phrase them as hypotheses, they were usually assertions about "the truth", but the fact of the matter is that they were essentially testable notions regarding which data could be gathered, and so they were hypotheses, whether intended to be or not. These philosophers laid intellectual groundwork for what would become modern economics, sociology, anthropology, and the modern social sciences in general. But we wouldn't take the writings of a 17th century "natural philosopher" as the final word on modern chemistry, so why do we so often take the writings of 17th, 18th, and 19th century social and political philosophers as the final word on human nature, despite the fact that we now have over a century's worth of data on the subject?
I think there's a couple of answers for that. One is that these philosophers all had agendas that are still at play in today's politics**. Adam Smith argued in favor of liberalizing economies for the good of society, Karl Marx was insisting that the downtrodden needed to be considered and not oppressed, John Locke argued for a model of government in which the people chose to be governed rather than having it imposed upon them, and so on. All of these ideas are still at play in modern global politics, and when one can pull out the writings of someone who was clearly intelligent and articulate, and who's name is considered near-sacred and above reproach, it serves well to further that agenda.
Also, all of these individuals had brilliant insights that actually did improve the general understanding of how humans function. Smith had an understanding of abstract economics that nobody before him could claim. Locke understood the structure of social hierarchies in the context of government in a way that few others of or before his time could rival. Marx grasped how the working class was impacted by the conditions of the Industrial Revolution in a way that few others of his time could, or cared to. But all of them were also very wrong on certain points. To take the examples of Smith and Marx: Smith failed to sufficiently account for the ability of those who were particularly successful in aggregating money and power via capitalist economics to manipulate the markets artificially, and he failed to grasp that the factors that influence the marketplace are so varied that models based on rational actors will ultimately fail to describe the marketplace in sufficient detail. Marx, by contrast, failed to grasp that, while class identity and power relations are important to the formation of group and individual identity, they are not necessarily the most important factors and will only motivate most people so far***. And this is not the end of the places where these two got things wrong. And outside of those two, don't get me started on why the "Social Contract" of is a nice idea but only abstractly defines the nature of government and the governed in a very vague way.
But, remember, these people were trailblazers. They were dealing with issues that others had not, and naturally they're going to get points wrong. That's not a knock against them, it's just the way that things work - the early thinkers will always get things wrong, which sets later people working on the same issues looking for better answers. As complex as the models of society constructed by these people were, the reality is much more complex, and people simply don't consistently behave in the ways that one would predict from the writings of these philosophers, even if their writings do hold many truthful elements. These men were writing based on their personal experiences and their own insights, but they did so with a relative dearth if fieldwork and data collection (yes, I know many of them traveled extensively to gather information, but they generally did so in a less systematic and coherent way than a professional researcher would), and much of the information that has been gathered in fields as diverse as anthropology and neuroscience since their writings has required the modification or even abandonment of their results.
Ultimately, this lack of disconfirming or modifying information may be part of the reason why these philosophers remain so popular. If you can ultimately boil government down to the social contract, or economics down to abstract market forces, or power relations to struggle between identifiable classes, then it makes the world seem clearer and easier to navigate. When I have had conversations with devotees of any of these people and pointed out the flaws in their arguments in light of the information now available, I have found that they simply re-state the argument without bothering to account for the new information. It becomes something of a matter of faith, and the advocates of these positions will tend to simply ignore or rationalize anything that doesn't quite work out (I remember one occasion when a Marxist told me that I wouldn't understand Marxism unless I was a Marxist...which is pretty much what many religious people tell me when trying to recruit me). That the arguments seem internally consistent and robust is more important than whether or not they actually describe the real world.
All of which would be amusing but irrelevant if it weren't for the fact that these philosophers are still cited and remain influential in modern political discourse. The end result is that, very often, politicians and voters put an ideology shaped by these early philosophers ahead of reality when it comes to shaping policy. It creates a huge potential for a lowering tide that grounds all boats.
*Cue the Marxists pointing out that Marx didn't believe in divine revelation in 3...2...1...
**Of course, you could argue that the reason, at least in part, that these issues are still in play in politics is that these individuals were so articulate in putting them forth in the first place.
***It's worth noting that the societies that really took Marx's ideas to heart were not the industrial nations where Marx thought that the workers would rise up, but rather the primarily agrarian nations where there was a much starker difference between the agrarian workers and the urbanized landowners and politicians. Of course, Smith wrote largely about an agrarian society (remember, he was pre-industrial revolution), but his ideas were largely taken up by industrial nations, so there ya' go.