6 Pokemon is too many for a team. You can't really ask someone which Pokemon are in their team and think, these completely mismatched Pokemon whose identities are half-consumed by their typing alone, are all cool and memorable in the context of them being the team of your bro, Mike.
3 is just enough to bond with, and places importance on deep knowledge of each pokemon's strengths and weaknesses, but type matchups being explicitly defined forces the number to increase for the sake of fairness.
All of those Pokemon trainers you encountered on the road with only 3 shitmons and a potion were cooler than any MC or champion.