But 'all' you have to do is track the pupil, and push focus on the object they're looking at. We're already rendering the focal length live in most games, there's no reason we couldn't shift the focal length affording to what the viewer's pupils are pointed at.
Alternatively, they could use a modified biology, where the direction the head is pointing chooses the relative DOF, basically you render a fairly wide depth of field around whatever the head is pointed at.
Anyway, you can get a pretty lifelike experience with just some basic atmospheric perspective. Look at photography with very wide depth of field [1], so everything is in focus simultaneously. You sorta just get used to it, and accept it.