/lit/ - Literature

I have an important question, divided in 2 posts.

1/2

Look at this shit.
If you are not an anime/manga fan that's ok, i'll explain: All these are manga and they all have the same stupid plot.
>Loser dies in modern world
>Some deity reincarnates the loser in a fantasy world.
>Loser has an ability that breaks the rules of common sense and works as a deus-ex-machina so he always wins in any situation.
>Loser suddenly has an harem of girls that fall in love with him even if he has no redeemable quality other than his god-like cheating ability.

They all even have the same "cheat" in the title and don't try to be original.

The question is: How the fuck is this not considered plagiarism? Most of these even take place during the same period, the middle ages!

They are all copying a successful story (sword art online i think?) involving a edgy MC that creates an harem with his cheating abilities.

What IS plagiarism?
How is plagiarism defined?
When does "inspired by x work" becomes stealing it?

Continuing on the next post.