It's also one of the situations where making a satisfying explanation is much harder than just not coming up with one. It's far easier to just not point it out, or use a very simplified explanation (in FF7, the party just straight up decides to go in small groups without needing to go into much detail as to why). This is one of the (few) things I'd say Chrono Trigger did wrong; it brings up that you can't go with 4 or more people, but then blatantly breaks why later on. Pokemon has an extremely simple reason; it's entirely arbitrary, it's just a rule/law that even the villains aren't willing to break. I've actually taken steps to just increase the party size instead of break them up, myself. One game I made eventually ended with a party of 7. Let me tell you, CTB solves so many issues of playing with a big team, but not all of them... The most recent game I made did the same thing as FF4 where your party changes by the plot and the player has little say as to who is in the party (sometimes there's an optional character that can join the party and it's up to the player to recruit them, plus the optional Lethal Joke character takes the fifth slot for the last third of the game).