I've randomly picked three to watch with my gf because one of her friends kept saying they were all master pieces. Castle in the sky, spirited away and Howl's moving castle

I did not really enjoy a single one of them, castle in the sky was the best but it didn't make me want to watch the rest of his filmography.

Just stop there then

I honestly can’t fathom how someone would not be mindblown by these three movies.

They all seemed to offer very surface level lore/characters, the themes are super simplistic too: war bad, nature good, consumerism bad. The rhythm is all over the place

It's not bad but it's a far cry from "these movies are all 110/100 and will literally change your life forever". I don't care for animes in the first place so idk, they might be that good compared to the rest of them

Nobody said they would change your life. They transport you to a different world for a few hours that’s it. Some of these adventures are quite epic as well.

Not all movies are for everyone. It's okay to not like something

It's also possible that a lot of people wouldn't like movies you love.

That said, Castle in the Sky in particular is not only one of the best animations I ever watched, it's likely one of the best movies I ever watched.

I thought Spirited Away (made in 2001) was awful, so much so that I didn't watch anything Miyazaki made after that for years.

I tried The Wind Rises (made in 2013), also found it terrible.

However, before those two films, I had seen several Studio Ghibli's earlier movies and found them pretty good. I thought Nausicaa of the Valley of the Wind (pre-Ghibli but the same group of people, with director Miyazaki) was absolutely amazing. So my impression is that their earlier films are much better than their newer works.