Not just japan but most of western world besides the US.
For various reasons the american culture puts high value in realism portrayed in art and works of fiction, and the "real" itself, consequently believing something that's stylized and imaginative as childish, never to be taken seriously.
Problem is the US has been influencial in that regard, so other countries picked that up as well. Due to US' use of the medium as something only for children, marketed as such, and the power it had over worldwide television the concept of animation has for a long time been sunk into the rut for many western countries, preventing many potential artists from exploring art that's either not realistic or some money laundering tool.
Animation is an artform. Art isn't just for a particular group of people, art isn't just for mature people, art isn't just high class paintings that you look at while spouting bullshit in order to raise your social standing. That has never been the point of art.
Art can be good, it can be bad, it can be obscene and disgusting, it can be everything and it's for everybody because it can come from anybody.
Art is true freedom and fuck you all who belittle and mock it.