It's so the reader can imagine themselves in the same situation. OR, so that you can mix things up by bringing modern knowledge/morals/whatever to a typical fantasy. The second one is so cliche that it's barely a functional trope at this point though, so mostly the first one.
Sometimes it's just used without thinking (ie cliche), or as a convenient excuse for why the MC is different from other characters in the setting- though that's basically just an extension of the above so...
Oh right, I mentioned this recently: Pioneer fantasy. You get to explore more stuff if you get isekai'd.
As much as I like this reply, I do think it's a nonsense answer, no disrespect to you, or the thought process.
Take Mulan for instance, the animation, not the god awful live action, where females are meant to shut up in their era, and simply be the mother figure in the back ground, serving their husband, or taking care of the family. Mulan had a different mindset, and different way of thinking without having to die, and without having to be sent into a new world, (although one could say jumping into the world of man, The Chinese Army, would be similarity of a new world..) Like I said, you can start a character with any mindset, or a different personality than any other character. Dropping that entire reason on, "Because they're from modern era earth" is lazy writing.
Sorry to say, you didn't change my mind.
Killing the character in the original world before bringing them to the other world is a sad JP thing. Because suicide is a thing and folks want to be away from the place (OG world) that caused them such grievance.
Also, death in OG world forces closure as the characters are less likely to go "oh, I should go back to my OG world" because already being dead.
And yes, Japanese do have this culture to just uproot and leave, known as
jōhatsu or evaporated people.
As to why start with such people, because reader association. This is to help readers and authors themselves to easily self insert into the work. For those who seek such fantasy at least.
@Cipiteca396 already wrote what I had struck through,
This is a great answer, which I personally relate to, having had my own issues on the topic. But it doesn't explain why it would be needed in cases of being murdered, or because of an accident. Not every Isekai starts off with a suicide. It's the necessary death in order to go into the new world that I think is dumb, not the new start within the story. Moving away, or leaving home to advenure somewhere new, can also give that sense of relief. Why death in particular?
You don't need to, but ways to get from 'real world' to 'isekai world' are limited for living people, most of the time its just going to be a summoning circle for regular transmigration and death for being put in another body.
But why have the "Real world to Isekai world" to begin with, why not START in the new fantasy world?
Isekai is indeed designed as a wish-fulfillment fantasy. This genre was born from the Japanese hikomorri culture, where many people isolate themselves socially due to the pressures of life. Why is the MC killed only to be sent to another world with a cheat? The short answer is because it's the fantasy of (Japanese) authors who want to escape their hard lives and go to another world to fulfill their desires. So it's no surprise that the worldbuilding of isekai is designed to be subservient to the MC. The stories are light, easy, and enjoyable. They even forget the logic of cultural reality, especially foreign languages. In essence, isekai MC = author (or readers) fantasy.
But why not start there, instead of romanticizing suicide? Are they advocating for suicide? Why not dissociate by living through the character beginning with the new fresh area which the entire story settles into after death?
Er, because you don't need to? I know it's depressingly common but far from all isekai kill their main characters.
One better question, which you touch on would be, why move a character between worlds to begin with? There are several reasons. Plain old escapism, reader substitute but, on a more constructive note, culture substitute. Drop someone from 'here' and you have someone who acts and reacts like someone from here (preferably).
We take a lot of sensibilities for granted. That's why we have all the 1980s and 1990s fantasy with farmer boys in a dirty world who never even heard the word 'education' before the story starts leave the village and behave like your average white male bachelor level academic from the get go. So, I guess, that's one point.
If you want to write a 'use modern knowledge to change the new world' then it's kinda hard doing it with a native.
The second better question would be why write a power fantasy at all? Because virtually every isekai (and a lot more sub-genres) are power fantasies. Because it's fun to read I guess.
Yes, which is why we pick up a book or read to begin with, for the escape, for the entertainment, to live in a new fantasy for a few minutes whether we hate our live or not. Why do we need the death at the beginning, when you're literally telling a great story immediately after, why not just start with the story? The point that makes an isekai an isekai is THE isekai. The dumb part. You're already telling a story; Skip the prologue of death, and it's still a story. Why not start there?
