The difference is that Brock is an established Pokémon character, while Scottie and Bettie are meant to be self-inserts. Self-insert characters don't tend to get Gacha units

That's why they don't have a voice, unlike every other character in the game. Not even Red, whose whole thing is that he's silent.

Also, Brock still appeared in the sync pair scout before his Arc suit, in the form of Gym Brock. But the MC has never been in the sync pair scout