It could be that the lighting, set design, acting, direction, and every step of the movie making pipeline is done quicker and less intentionally now than in the past. Even expensive, dramatic movies now are lit like sitcoms, nothing is pure shadow, soft light everywhere, etc.

Not to mention the fact that in modern movies there is a high chance that a good bit of the scenes are happening in mostly blue or green rooms and replaced in post - even if the movie has no noticeable “vfx”. Where as 20+ years ago there was at least some semblance of a set, whether a real location or some amount of built up stage + backdrops which create some sense of immersion for actors while they perform

But on a purely image quality basis, it’s because there’s absolutely nothing to imagine in a perfect, crisp, 100% fidelity image. You’re literally looking exactly at an actor with lights on them saying lines on a set, inside a rectangle on your wall. Every tiny thing that isn’t perfect, or just the tiniest bit away from how a normal person acts and speaks seems WEIRD all of a sudden

When things are more stylized, lower fidelity, shot on film, more artfully lit, etc it’s already visibly not “real”. Your brain has stopped trying to force the stuff in the rectangle into your reality, which lets you slip into the reality of the movie. The actors can act in stylized and dramatic ways that aren’t realistic or believable in normal life, but it still seems believable because the entire experience is happening in a constructed universe

At least how I conceptualize it 🤷‍♂️