Moreso responding to the part I bolded; it were due to the fact that children will be caught off guard with having to suddenly face six Pokémon at once as soon as the first Gym. I do not think older but still casual fans would be prepared for such a jump, either.Adding to this, there's a reason why the vast majority of fan-works give all the bosses full teams of six mons; it allows for far more memorable and actually tense boss encounters that challenge your skill. Just look at the first Gym battle of Reborn for an example:
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Obviously, this isn't exactly threatening, but for a first "major" boss fight? This works. It actually asks you to consider things like team composition and set-up, rather than just being essentially an enemy trainer with slightly higher levels. And that's....fun? It's actually making you meaningfully engage with the game? Dunno why that's such a controversial stance.
If anyone explicitly states that the player should bring a full party of six for Gym Battles, it would give a foreshadowing that the Gym Leader will have a full team, too, so that the player will be prepared for this fact. Just as early as the guy who is at every gym entrance, or the receptionist, telling you in advance that you’d better bring a full team of six will help warn the player about this.
I do agree that Paldea’s Gym Leaders didn’t have much personality in the long run, while Paldea’s Elite Four managed to be more memorable due to interacting with any named character than just the player, namely each others. At least there are a few gems gameplay-wise, such as Iono’s Tera Electric Mismagius who have Levitate to negate Electric’s only weakness in ground.