When I used to read for pleasure, I did it because it was pleasurable. Not because it would be the hard thing. It was fun and easy.
What this particular chain of thoughts shows is that adults don't read for pleasure either, they associate it with an uncomfortable hard thing one should to do "build character".
This is conflating hard with unpleasant. A child just learning to read is going to find it hard to do, yet through adults pushing them to do the hard thing, they learn to read and sometimes begin to find it to be pleasurable. Building most skills is hard, yet that doesn't exclude taking pleasure in it. Many of us taught ourselves to code, the fact that we enjoyed it doesn't mean it wasn't also hard.
We've all learned the lesson that sometimes you have to struggle through something hard, to be able to access better pleasure.
The original donkey kong is pretty difficult compared to some of the wide-audience games that have been coming out. As far as I can tell, if the audience of a particular franchise includes younger generations as a majority or near-majority, the difficulty plummets. I don't think "plummets" is even that sensational. See pokemon, kingdom hearts, mario games, final fantasy games. Some franchises and genres have survived but not all of them.
I might be missing some other reasons why this could be happening, like increases in game balance and coordination.
Play Mario Odyssey for an hour or two then play Super Mario Bros 1, 2, or 3 as one startling example.