What we lose with over-exposition is a crucial narrative distance and the suspension of disbelief. Instant gratification is ruining storytelling, obviously.
Stories are at their greatest when they believe they know what you want, and then they keep going. (Published art is mediated by the expectation of an audience, you cannot pretend that it is just one fucked up mind executing a fucked up idea.) If the creator is doing the equivalent of sitting next to you and explaining their reasoning just in case you missed it - that closeness to the viewer reminds you that this is a story, and once it tells you you've got to understand the story a particular way, it dedicates less narrative space to the smaller stuff that might extend and bring nuance to the story, and it can rob audiences of the fruitful exercise of analysis. When service to the viewer supersedes service to the themes, we get declining media literacy and just bad storytelling.
A narrative world is most effective when there's a sense of richness - like a painting: if you stopped focusing on the most salient elements, you would notice an aesthetically coherent background, less detailed but undeniably present. You could look away but the world would go on, it would hold itself up.
Not every show can be Nirvana in Fire, obviously, but I think it's a particularly elegant example when thinking about trust in the audience. Whenever Consort Jing converses with the Emperor, you can understand that she led him to that specific conclusion, but you can't quite pinpoint how. She's a side character and she doesn't need to be anything but a side character - the crux of her characterisation is her subtle navigation of court politics. This would all crumble if she explicated her reasoning, and it would distract from the main plot. This is how you build depth with restraint, this is structural soundness, this is the creator-viewer contract. You'll believe in a false world if the creator makes believing easy.
TLDR: When a story starts explaining, it can't stop!! The creator may as well hand you an essay.
