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".
If you want to make the argument that it's muscle memory in e.g. shooting freethrows in basketball, then you can see the exact same thing in doing chess tactical puzzles. There's even one successful learning method called the Woodpecker Method where you endlessly repeat over the same series of tactics working to get the time it takes you to do them down to essentially instantaneous. And it works excellently for improvement, and I obviously don't just mean improvement at doing that set of tactics.
We've all learned the lesson that sometimes you have to struggle through something hard, to be able to access better pleasure.
You did not struggled and it was not hard. Kids books were written to be easy at age they were targetted at.
If the issue was just "hard" videogames woulsndo the job.
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.
In the world of games outside the big money AAA MBA stuff, there's plenty of highly challenging franchises that maintain true to themselves and thrive, even with plenty of kids playing. E.g. - I suspect the median age for Binding of Isaac is well below the age of consent.
One of the things though is when most peopley play SMB 1-3 today, they're playing with input lag. Mario Wonder was designed with input lag in mind, SMB 1 was not and it increases the difficulty.
Mario Wonder lets you choose to use invulnerable characters, etc. There was only one level I remember needing to try many times to beat. OTOH, there's lots of difficult levels in smb 1...