The short version is, an OLED/LCD has an "infinite" focal length and cannot do otherwise. So by default everything in the display is rendered at the same focal point. You can do some tricks like blurring elements, but these are not responding to the convexity of your eyeball or how your eye muscles are working, so if you look at something that is software blurred it will still look blurry. Even with eye tracking it's still really a hack, and isn't naturally responsive - it's the software reacting to your gaze.

Display based accommodation however renders objects at different focal points. So when your eye changes gaze point and your ocular muscles respond to put your eye at that focus level, the object/s come into focus. It's the reverse of the hack.

Sounds like a trivial difference but is completely different, significantly more convincing and logistically more complex.