
idk. maybe its because they feel like they're stealing the show of the "original" MC??
or maybe because they've already read the novel before getting isekaid, and they know that in the story, the ML lived happily with the original MC. so since the isekaid characters grew fond of the MLs, they just want the best for them so they think that the ML deserves to be happy in the end? (which is to be w the "original" MC?????)

it's quite a realistic choice tbh. If you know that there is a hole ahead of you, are you going to take a long way round or choose to jump to the hole and stuck there?
In this case, replace the hole with the perfect couple you read in the novel...You read a whole novel about how perfect in love the couple is, it's normal that you will be biased over the flow of the story. It's a cliche, but that's probably because all of the manhwas lately are all about reincarnation into a novel. If you don't read a different genre (no reincarnation), you will see FL with different choices :))

tbh, I would jump there. If ML changes his feelings after meeting FL then fine, it meant to be, but if they are currently interested in me and I like them as well, I don't see why I have to push them away. What will happen in the future, I don't know, but I know the current one is me and that's enough

I mean even if they know the ML live happily with the original MC, the fact that they are here and ML currently in love with them is a fact. I can't imagine myself looking at someone who is in love with me and keep pushing them away and telling them "You will meet someone better in the future, guarantee. Or you will life would be much better if you do this and that", while denying their feelings and what they said. I never like the troupe: " I'm doing this for your happiness".
If after meeting original main character and ML changes, then well, it cant be helped, but not listening to what they said and pushing my own agenda/ what I think it's right on other people, I just don't like it. Sorry for writing such random, all over the place comment

The same logic when the FL reincarnated in a story where she receives the death flag. She will proceed to avoid that. It's human nature. If you don't like to see 80% of the FL do that, please choose a different genre that doesn't involve reincarnation or isekai in the list.
In this case, the FL knows that she could die by being with him (her sister almost died) - being with the guy with a lot of assassins followed, and live in the house where everything she touched turned into monsters is dangerous.
Second, she also knows that there is a story's FL out there and she read about their loves before reincarnated. If you know a happy couple in real life (and you read about their happy-ever-after), are you going to become the third wheel?
Third, she knows that ML's curse can only be undone by the novel's FL (which is stated as the only way in the novel she read). Without that, he will be constantly exposed to danger if he's in his child' form. Again, the human's instinct to protect their loved ones here.
Please don't apply the overly-romanticised POV of reincarnation fans by judging the plot as being too cliche. There is a reason that 80% of FLs choose this route. Just like Killing Stalking fans are all fixated on a romantic relationship between the two main characters while the authors themselves have clearly stated that there is no romance in the story. Just psychological and horror.
Why do MC who got isekai-ed as side/villain characters are always like this :" Nahhh, he/she will be happy like the book."/ "I'll have to leave them be cause they will fall in love with this and that character." Like don't they see their partner love them ? Like most of the story, they know that FL/ML love them, but they still neglected their partner's feelings. Why ?