I find it incredible how people can eat fish without even knowing what a real one looks like-- mind you, that is probably why they are okay with eating fish in the first place!
Catching fish for "sport", on the other hand, is more a matter of desensitization. In other words, many people start fishing at a young enough age to never even question hurting the fish, or it's just that their parents tell them it is okay. Children will do a lot to get a role model to look up to, so they may even change their beleifs in order to accomodate for their parents' ones.
Please don't even get me started on catch-and-release fishing-- what a cruel way to kill a fish. The fish who you throw back will end up being weakened by having their scales rubbed the wrong way, will struggle to eat after having their mouth being cut, and will have gone through the trauma of being half-drowned in the air. Their blood (from where their mouth was cut) will also attract predators. Their chances of survival will have been severely reduced.