I actually know why this is! Prior to about 1997 or so, the prevailing idea in children's entertainment was that it needed to be "whimsical" to stoke imagination and curiosity. This is why a lot of children's movies and TV shows from the late 20th century put tons of effort into writing and animation (
some more than
others, of course), because the people making them assumed that was what children were interested in. This is why a lot of those shows, books, and movies are still remembered fondly and enjoyed by full-grown adults today.
In the late '90s, though, what was
actually discovered in the field of child psychology is that kids prefer media that's scientifically developed and adjusted to their immediate wants – i.e. lots of colours, context-free noise, and repetition. This is a hypothesis that was tested several times through shows like the Teletubbies, Barney, and the modern incarnation of Sesame Street, which have teams of researchers behind both the development of the shows and the writing of every episode.
It's been academically proven that this new kind of media is better for helping kids develop motor skills and capacitive memory, so, objectively speaking, it's "healthier" for them than something like The Secret of NIMH. Of course, the downside is that they have precisely zero appeal for adults, but there's so much money in kids that it doesn't matter, and, well... here we are.
People who were born in the 80s and the first half of the 90s grew up with very, very different children's media than what exists today, and their expectations are wildly-different from reality.