because transmigration is used in many stories to i) follow the trend and ii) have a reason to portray the character as competent because readers generally like competent characters
there are quite a few manhwas out there where the mc has transmigrated/turned back time and this plot point literally does not matter to the plot beyond the first few chapters where the story is being set up. it could have been replaced with the mc going to an accurate fortune teller and getting their fortune told and there'd be no change to the plot.
many authors are only including transmigration in their manhwas because it's a trend, they don't actually incorporate the transmigration aspect well into the plots, characters never seem to actually care about the life they left behind or struggle to adapt to their new life and identity. characters being dumb and a damsel in distress have nothing to do with being transmigrated and all to do with the author not knowing how else to introduce conflict in a way consistent with the character, because you'll see this in non-transmigration stories too
tl;dr it's bad writing
That my pet peeve too. Specifically the part where the plot take 5 panels to say “I was transmigrated into a book i read before i died to a side character that is bound to die soon” and then proceeds to completely forgot about that plot. Whats the point of adding it? Like srsly what? It makes the story soo cheap cos its soo unnecessary and often times its not even used afterwards. They dont use their modern knowledge to navigate in their problems later on (and if they do its like overextended plot arc like business strategies) they always follow the plot of marrying the duke or marrying this person etc.
Also another is the sheer speed the character is able to assimilate and adapt to their new environment. Like how r u not getting depression or withdrawal symptoms how are u getting so accustomed to carriages, to medieval living, to magic, to etc. for example one that I absolutely detested was the mc transmigrated (not died) to a duchess/ counsel of the muderer emperor who woke up in the middle of an execution. Lile how tf can u stay calm in that time not get scared depressed or even look for ways to go back? Lile mc just went with the follow like its natural and guess what the transmigration part didnt matter after cos she fell for the emperor. What the absolute shitshow was that
Why do isekai manhwas, especially the ones where the MC is transmigrated to a book and becomes a character forgot halfway that they're not really the character?
Like that's one of my pet peeve, at the beginning they're strong willed or very eccentric but as the chapters go by it's like they don't remember they weren't actually that character? That they can actually be smart and brave to survive some scenarios and not depend on the ML or other supporting characters.
I mean, it's understandable that they have to method act and truly internalize to survive but it's just frustrating because a lot of times their original personality is smart and strong willed but they suddenly becomes dumb and a damsel in distress, like what???
Also, as they go towards the ending, suddenly they're the original again.