I couple thoughts.
When fishing for flats, even if they used their vision more, the size of the line relative to the bait is really small. Which do you think they would focus in on? Would they even notice?
When fishing for cats you are on the bottom and the line from the sinker back pretty much hugs the bottom. In that case, will they notice the line? Could be any debris sitting on the bottom. Line visibility is more a concern when pulling a bait or dropped down underneath a bobber and it becomes more of an issue the smaller the bait/lure is…IMHO
Lastly, since fish have no concept of a line or hook, maybe line visibility just represents danger because something just doesn’t look right. Or if a fish has been caught before, maybe then they actually “recognize” line as danger, even though we’ve been told fish have short memories.
Or maybe the line actually becomes the “profile” of the lure or bait and the fish think that creature is too big too eat or would be too difficult to eat.
Maybe I am giving fish too much credit.