Is that really true? I find that more and more TV series have a lot more gruesome elements in there nowadays, also ones aimed at not-only-adults like Stranger Things for example. And horror movies moved from being a niche thing to Hollywood.
Eg. BBCs Black Mirror - which is obnoxiously turning out to be more of a prediction then a cautionary tale - is definitely in the range of "darker content" in the vain I was talking about. But the target demographic is adults.
The old stories are meant to relate to you. The whole reason they were being told to very very young kids (<6yo) was to make them understand the unfairness of the world in order for them to hopefully be on guard against it when it matters.
They also weren't really fantasy in nature, even if we consider them to be fantasy nowadays. And that's a big part of why a series like stranger things feels inapplicable here - unless I miss remember it's setting. Wasn't it fundamentally just entertainment? More about spectacle then actually relating to the viewer?
The young adults/teenagers are both 10+ years older then the target demographic of the old tales, and won't relate the story to themselves because it's too disconnected from reality? At least that's my impression.
Think about all the serial killer and urban crime movies in the ‘80s and ‘90s, or the film noir of the postwar period.
TV is more complicated, but cop shows like the early seasons of Law and Order and all of SVU, NYPD, and then later The Wire and The Shield were pretty gritty.
Video games have always been a mix of squeaky clean Mario and Zelda and gory content: Think Doom, Mortal Kombat, Grand Theft Auto, Postal, etc.