the point for them for making same fish but renaming was for storyline thing as aht urghan, nowadays its pointless, yes, however back then it wasn't.

don't forget alot of items are used for people skilling up which people still do.

as for deleting being easier than creating, the answer is no it isn't easier, infact its harder because your dealing with a massive game.
When you want to delate a piece of code, you gotta removing all the links to that code and redirect it to somewhere else. you break the fish, the ingredients, the crafting, the information, auction houses, NPCs, any attached missions such as campaign ops. coffers that drop it, mog garden, current stock on people, not to mention all values,locations,etc,etc,etc. you have to make sure you get it all done right or it'll cause errors, potentually massive errors, for farming, fishing, crafting, eating,battles, and even the auction houses, etc etc etc.

Creating something just means you make new links, so your free to decide yourself, breaking something means you need to redirect all the other code connected to it, while they probably have a program for finding most connects, if theres 1000 links/connections for 1 item, you got alot of work ahead of you, let alone removing 50-100 items totally.

however if they decided to block a item, thats a different story, since all links leading to that item imediently can be redirected somewhere else or the game recognises it as not useable or unseeable