While cinematography isn't the whole answer, I think it's a big contributor to why so much feels aggressively middling these days.
Modern shows are aggressively aesthetic. There's huge overuse shallow depth of field, resulting in blurry backgrounds. Watch modern movies with this mind, and you start noticing the reliance on a sharp foreground and blurry background is extreme to the point of bizarre. Cinematography should be a tool to achieve an effect; blurring the background to intentionally make the subject stand out is a purposeful use of that tool, but is being applied everywhere now, with no intent behind it, even if it's "aesthetic".
I would also argue that it's easier with digital photography to create blandly attractive, "painterly" images, thanks to colour grading and increased dynamic range and so on. A lot of shows these days have technically competent photography, but it all converges on the same aesthetic — tons of diffuse, lush lighting (often achieved with filling the space with lightly cinematic fog) and impeccable set design, and that creamy depth of field. But there's no contrast anywhere, it's just creamy "aesthetic" blandness in forgettable environs.
Another non-visual aspect rarely mentioned is audio: Almost all TV/movie audio these days is foley, and it's sometimes jarringly bad when you start to actually pay attention to, say, the sound of footsteps or keys jangling. High quality productions can be very good here, but most productions don't spend enough time on it. Bad foley has a very strange, subliminal effect on a scene, further undermining the sense of reality.
In such environments (visual and aural), nothing seems real and nothing seems like it matters. Everything, even nominally "adult" shows set in the real world, feels like Midde Earth and not Planet Earth.
It's not all bad, of course. There are also definitely cases where the quality of the show transcends the mediocrity of the cinematography. We are still getting good shows and bad shows, like always. But it does seem like things have shifted into a sort of middle where everything is average in the same average way.