Unfortunately there are some real problems with that. Imagine a pair of regular glasses with a small dark rectangle on the lens. Do your eyes see the nice sharp edges of the rectangle? Nope, they just see a dark blob because it's too close to focus on.
In the same way, even pixel perfect darkening has the same problem. You don't see a nice cutout, you see a blurry blob.
Why is a blurry blob that big a deal? In theory a headset could use the focused additive display to draw passthrough video in the blurry regions that should not be darkened to provide a crisp blackout edge at a natural brightness.
I think that would need to be adaptive to the current focus of your eye when not looking at the display since we're talking AR. That's a much, much trickier problem than delivering light of constant focus in a workable AR package, which itself is no walk in the park.
There's a difference between projecting focused light into the optic path and blocking the natural light you're selectively trying to allow through that same path.
In the same way, even pixel perfect darkening has the same problem. You don't see a nice cutout, you see a blurry blob.