>>52126563His team, regardless of it being GSC or HGSS, follows a trend, specifically in Yellow.
Pikachu is the starter. the three starters are all Gift Pokemon on the way, so is Eevee and Lapras, and Snorlax is a mandatory battle.
Mewtwo, alongside the legendary birds, are optional battles, and therefore don't fit the same mold of being story beats.