@MiiaBestWaifu it's a matter of immersion, not realism ("science").
When you're reading fiction, a lot of things will be different from real life, but the reader is supposed to accept some of them as true for that work, in order to enjoy it. This is called immersion: accepting unrealistic stuff for the sake of the story.
For an isekai, being summoned to another world is one of those things, so it's often well-integrated into the rest of the story, in order to make it feel natural, and avoid questions like "wait... summoning doesn't exist" to break the immersion.
On the other hand, "OP katanas" are not like this. They aren't a necessary part of the story, and most of the time they pop up in fiction is because the author believes in the myth they're the strongest sword out there. It's unnecessary and a lot of readers know it's a myth, so it brings up that "wait... katanas aren't that good" that does break the immersion.