I think we disagree. "The enlightenment" and periods/movements that followed it were ignited by excitement and optimism about what science is and what it can achieve. Science is a bit of a broad term and it doesn't exactly mean what I want it to in this context.
Marx is a good example, he strived to (and claimed to have) put forward theories on politics, history, ethics & economics that were the somewhat like of Darwin's theory on the origin of species. That they could be validated and built on in the same way. Further theories and technologies could be based on them in the way that rockets can be built based on Newton's mechanics.
The left isn't unique here. Ayn Rand is almost a caricature of this point, partly because she actually deals with modernism as a subject matter. She claimed to have described a theory of morality derived from pure logic and politics derived from morality.
There are some fields where "scientific" endeavors produce some outcomes, but have never really driven the kind of progress that was hoped for or claimed. Psychology and economics are big ones, IMO. Early economists like Ricardo could pose theories that work logically and are somewhat available for experimentation or prediction-measurment, but the kind of theories produced by modern economists like Marx, Keynes or Friedman (to be balanced) are not testable. They also haven't really produced any decent technology based on them or a reliable understanding of the mechanics.
Whether or not "the humanities" or the areas between humanities and sciences are amenable to scientific investigation is a big question. I'm not confident in any clear answer but I suspect the truth is that there's a spectrum. I am pretty confident though that the subject matters have been, up until now, for less amenable to scientific investigation than biology, physics, chemistry, etc.
If I had to bet on a solution to addiction, I would place a bet on either a neuroscience approach or a totally nonscientific approaches (like AA or religion) over psychoanalysis. I have almost no confidence in the idea that a theorist will figure out a better form of government. The process that will drive these things will probably continue to be cultural emergence.
I'm not saying that humanities are useless. I'm a big fan of the humanities. I'm not saying that science is bad either, I'm a fan of these too. I am saying that modeling the sociology or genera sturdies departments on the physics one turns out to have been a mistake. It hasn't been productive and it doesn't produce science or its derivatives.
I think many of Marx' ideas are valid, particularly his definition of class. I think his (and most modernist "political scientists") insistence that his theories on history or political progression are based on logic and science or that they can be is like a monarchy's insistence that they are chosen by the gods. Bunk.