It's possible that the vision system evolved long ago under radically different circumstances and just had no imperative to get rid of it.
On the other side of the equation, vision is so stupidly useful that it has evolved independently from several different systems in different organisms.
For example, animals who end up in dark environments lose sight relatively quickly in a few generations, but for other cases, do animals tend to lose specific parts of their vision system - color, edge detection, etc.?