Poor planning. It has to do with the coding and how the game recognizes the fish. Basically the aspiration is coded in such a way that it specifically needs the basegame fish.
To be honest they probably should've made it a % anyways, because if they did take your advice, that step of the aspiration would be a cakewalk and only get easier with time. A % ensures it remains decently challenging regardless of the amount of fish.
I personally wish the bait system wasn't so terrible. Fish in Sims 3 loved specific types of bait, making them easy to catch if you did the work to get their bait. Sims 4 has the problem where the bait types are things like "FRUIT" or "SMALL FISH" or "TANGIBLE OBJECTS." The result is you cast out your line and 80% of the fish want to bite it, and then much of catching the right type of fish in this game is based purely on luck.
