They differ because his rival is abusive towards his captures, using pokemon like Salamence to defeat Caterpie's. This is another message from the producers that catching pokemon without battling is correct and more humane.
More seriously though, I don't think that a classic rival can work for Goh's goal like it does for Ash's. Ash needs to overcome his rivals to move forward (not necessarily the case in this series), and in this series a loss can mean regressing. Failure on Ash's part usually means resolving himself and working hard to overcome the obstacle.
Goh doesn't have a regression condition, unless his pokemon are stolen or leave then he only moves closer to his goal. He hasn't shown the desire to replace his evolved pokemon, and failing to capture a pokemon isn't like an tournament opponent that stops Goh moving forward, as he can always catch other pokemon.
Anyway, the show seems to be moving away from random encounters to Goh actively seeking out specific pokemon: Pikachu, mystery pokemon (flygon), A-Eggecutor, a fossil. Just have him fail some of these quests for whatever reason (pokemon training, etc..), and have him work on it.