i have seen this called the "D&D problem": when you have a large cast of disparate characters going adventuring together, how do you actually make it make sense for them to stick together?
the classic case is "why the fuck is the lawful good paladin traveling alongside this chaotic neutral rat bastard sorcerer"-and while you can come up with plausible near-term rationales for this ("the town you both were in was suddenly under attack, and thus you banded together to fight a common enemy"), it's reaaaally hard to stretch some plausible reason over the course of an entire adventure.
in D&D, folks tend to just kinda "go with it" because, on a meta level, we just wanna play a goddamn game with our friends.
in JRPGs, we are often also asked to kinda "go with it", but to varying degrees involving various slights-of-hand, and i find it super entertaining to squint at games and figure out, how much of a fast one is this game trying to pull?
my favorite case is Chrono Cross. there's a point, relatively early in the game, where you honestly don't have any incentive to go on. you got warped into a crazy alternate dimension, you figured out a way to warp back to your home dimension, and you saved the life of the random cute chick who helped you out while you were in alternate-dimension-land. you can just go home now!
but random-side-character Macha says, "hey, aren't you curious what that sketchy villain dude's deal is?" you can choose yes, and go track him down. or you can choose no, and... Macha proceeds to chide you for LACKING A SENSE OF ADVENTURE, and then you can choose yes or no again, until you finally just choose "yes" and go on an adventure.
and all your party members-all of them! just kinda go along? usually for "reasons" that are explained in literally one throwaway sentence. it's wild and silly and you're definitely not supposed to think about it too much, just go with it.
so yeah, Chrono Cross gets an F grade on Party Coherence Factor.
off the top of my head, here's grades for other random games:
FFXV: gets an A-, surprisingly. the plot's sort of a chaotic mess, but not in ways that drastically affect the believability of "these people would in fact join together to do these particular things"-it's a king and his royal-guard-slash-childhood-friends; of course they're sticking together. the ding is because it's never entirely explained why collecting all the astrals + royal arms is a higher priority than, y'know, anything else you could be doing to save your kingdom, but i think the assumption is that we're going by Anime Power Level Rules and everyone understands Noct needs over nine thousand to beat the big bad or whatever.
FFXIII: also gets an A-. as awkward as the whole l'Cie mechanic can be, it does present a very convenient forcing function wherein everyone involved is real damn motivated to stick together, because everyone else wants to murder them. i do vaguely remember finding it a bit surprising that everyone was so on-board with the mission to rescue Sazh and Vanille on the Palamecia, but everyone was kinda buddies by that point so good enough.
FFIX is like... a B? it's very coherent in the early part of the game: while not everyone has the same motivation (which is what FFXV and FFXIII had going for them), their motivations very neatly align-Steiner and Zidane are both determined to protect Garnet, Vivi wants to learn more about the other black mages and that seems to correlate really well with wherever the fuck the others are going, Freya's hometown gets obliterated fairly early on and her boyfriend has Tragic Anime Amnesia so it's not like she has much better to do...
but it gets a little tenuous later in the game. for instance: when it's demonstrated that Kuja is a real and potent threat, it sure seems like we should be sending a whole goddamn army to the Desert Palace rather than just Zidane and his ragtag buddies. also Garnet's insistence on not telling Cid that we're off to fight God is maddening, etc etc. basically, once the conflict goes global rather than strictly personal, it seems like it should have more of a global magnitude, but-hey, just go with it! it's fun fantasy adventure, making it global would be strictly less fun :P
anyway at this point i got bored of assigning ratings, but, feel free to share your own Party Coherence Factor scores in the comments or w/e