not that i’m at all an expert about movie tie-ins or know enough about the other movie tie-ins you’re talking about to be making a general assumption here, but if i had to guess, it’s a lot about the nature of how a movie tie-in connects to the rest of a series (or the attempted “lack” of a connection).
there’s a pretty consistent formula to yugioh: yugi (or jounouchi) duels someone, the someone has a problem, and the duel helps resolve or otherwise address that problem in some way. character development via the duel.
now, we could have just had a movie where this formula exclusively follows yugi and kaiba’s grief. this would have been great and i’m sure everyone would have loved it. it wouldn’t NEED the card game, but by virtue of kaiba’s presence and emotional constipation there is still plenty of opportunity to explore that grief literally directly via dueling. in fact framing it through that would give a lot of creative opportunity to dig into exactly how these characters are doing mentally and emotionally, through their relationship to dueling and each other, and also through plain old card symbolism (à la gandora).
the thing about movies in relation to part of a franchise in another medium (manga, anime, etc.) is that there’s this idea that it has to be able to stand alone. most don’t just by nature of…being part of a franchise, but they still TRY. so we couldn’t have it be entirely based on yugi and kaiba’s grief, because getting into that in more detail than what was in dsod involves knowing more than someone who only kind of knows what yugioh’s about would have context for. and of course if we’re dropping a movie, we have to make sure we get as big an audience as possible, right? so we have to cater a little to the non-fans, too.
(also, if i’m being honest, i don’t think most of the writers were more than surface-level interested in yugi’s grief. he is so painfully not the focus of the movie that i feel like i’m forgetting things that didn’t happen. he is literally the protagonist. i don’t get it.)
so a casual viewer won’t have enough context for a movie just about our beloved characters. but you know what would help those casual viewers get some context and bring them into the story with some grasp on the emotional stakes?
that’s right! it’s the director’s movie exclusive oc! 🎉
basically, the structure of a movie allows for the full development and resolution of a character arc, so they stick a New Guy Nobody Cares About in there and have way too much focus on them in an attempt to make people care about them. because admittedly you do need to do that if you want the character arc to have any kind of emotional impact. it’s just that dsod had much more interesting things going on that were not focused on as much as they should have been.
but generally when a franchise does that they want as many people to watch it as possible and also want it to be ✨unique✨ from the rest of the series because it’s a movie. and also they need it to have flashy duels because this is yugioh and they can’t do that with the characters we already know the playing styles of apparently.
so. yeah. that’s my best guess on why that always seems to happen.