The "culprit" here is the cold fact that fiction needs to maintain a high signal-to-noise ratio in order to work for most readers, where 'signal' is immediately relevant story content and complicating or background details are classed as 'noise'. While there are people who would read Kim Stanley Robinson for his infodumps, there are lots of other readers who want immediacy, immersion, and tropes and patterns they can recognize. (A parallel problem crops up all the time in games if you try to front-load your tutorials. People want to play the game, even if that means you need to teach them on the fly.) Readers want story, and they want it now, and they don't want too many characters to confuse them, or a lecture about the history of kingdoms X & Y.
The trouble, of course, is that often "don't want too many characters" means the cast gets pruned down until there aren't any servants, or merchants, or people making the kingdom run. The trouble with only slipping in history through evocative details is that not every kind of history is amenable to being boiled down to easily referenced soundbites. And so on.
Also, when we write stories, we don't get to cover even a fraction of the issues that are relevant to a particular world or time period. We rarely get to delve deep into technological innovation in stories centered on court politics, for example, and when we're following an outlaw and his nemesis on a chase to a hidden tomb, questions of public health won't usually get much page space. That said, I feel like this creates an even greater responsibility for authors to think through the implications of the choices they make and the aspects of their world which they depict. I sympathize with the desire to cram more things in, and the temptation to make them simpler and easier for people to wrap their heads around, but... I guess I feel like the pressure to simplify and flatten complex issues is already massive? The standard narratives of our culture demand clear sympathy and easy answers, and sometimes that's okay. But it's not okay if it's all we produce, or all we promote, and I get uncomfortable when people make claims about story that implicitly or explicitly denigrate complexity, and exalt emotional appeals over intellectual ones.
Anyway, like I said, half-formed thought. I'd be curious to hear what other people think about these issues, as I feel like there are a whole knot of them in dynamic tension with each other.