Anime

It's not that deep. They got the rug pulled from under them when the studio slashed the budget. Instead of doing an ending they wanted, lots of action, with bad animation, they instead rebelled with a rewrite ending that used intentional vagueness and ambivalent philosophical storytelling that let them use their resources to focus on art design and keep the cool factor up. Tons of stills, but really pretty. That's why a lot of the last sequences are just artsy backgrounds with only voice overs, there are several seconds of just white paper, etc. This however proved widely popular and the writing was vague but interesting enough to fuel hundreds of hours of video analysis and thousands of words essays.

The other factor was the mythological treatment of biblical imagery, which was rather new and unique at the time.

Eva starts throwing out wild amounts of lore and background information out of nowhere and never really stops to explain any of it. I don't think most anyone fully grasped the show on their first watch through. In fact, iirc a lot of what was figured out was with the help of a Playstation game that came out that had large text entries of lore.

It could be thought of as deep. I remember it being meanigful to me. A visual representation of the ego dissipating and watching the main character with numerous neurosis honestly reflect on his life and character while communicating with his subconscious that's taken the form of the people in his life. Seemed like it was what the whole show was working towards.

I have a simple explanation that's more Doylist than Wattsonian:

Anno is kind a dick.

Evangelion is a long series of borderline rug pulls, culminating in possibly the most infamous rug-pull that's not just "it was a dream." There was never supposed to be a textual climax and payoff. It was always scripted as a suck-zone for you to care about the characters so you could appreciate the introspective what-ifs in the last few episodes. Like if Tolkien stopping halfway through Return Of The King to say "fantasy is bad, actually." And then spent six chapters litigating the dynamics of Frodo and Sam's relationship, because that's what all this Middle Earth bullshit was really about.

And then the movies and the rebuild are various forms of Anno targeting critics of all perspectives and finding genuinely creative ways to say "fuuuuck yoooou." Like, thematically? The "curse of Eva" is ingenious. But the whole exercise is a giant middle finger to anyone foolish enough to care about the story he's telling.

It's satire that doesn't work. It's too functional as a sincere example of the genre it's aimed at. If the man had been dragged from the studio, kicking and screaming, somewhere after episode nineteen and LCL splashing against a van on the road - if the last half-dozen episodes had been rewritten to play the story out, to its extremely final conclusion - it would still be revered among giant-robot anime. Its influence would be nearly identical. But all the philosophical payload that needs two left-turn episodes to "explain" would be known only to one pouting young director and a few co-writers. Vanishingly little of it is in the work, three-quarters of the way through the work. God help us, if the wrong hired gun took over, "get in the robot" might've been Shinji's major character arc.

Contrast how The Last Jedi leaks "anyone can be a hero" from every seam, despite the finished product being roughly hammered back toward the status quo. Half the parts people like and most of the parts people hate are beautifully attuned to that lost moral. Meanwhile Eva can't get people to go 'ohhh, escapism is bad!' even with two episodes beating them over the head, and a film sequel that spits on everyone who saw it, and a remake that loathes its own existence.