Because it's less about the specific Pokémon and more about the actual situations that used to cause him trouble. Having him meet the exact same Rattata and Fearow would feel a bit more forced towards the fanservice side than to show the actual character growth. The Pokémon being the same species is more than enough if you want to make clear parallelisms.I don't get it. If the writers wanted to show Ash's growth as a person by befriending Pokemon that have caused him grief in the past, then why use these one-off stand-ins? Have him actually befriend the Fearow that attacked him in ep one. That way it'd be a perfect bookend to the series.