This is only due to current screen limitations. If someone can pull off a practical light field display, focus and convergence should work together just like they do in the real world.

Real light field displays have already been demonstrated [0] [1]. As another example, the hype around Magic Leap seems to be largely due to some novel light field display technology using an oscillating optical fiber as a light field scanner.

[0] http://alumni.media.mit.edu/~dlanman/research/compressivedis... [1] http://www.computationalimaging.org/publications/the-light-f...