@_hng_ it's not that Japanese see katanas as OP, rather imo it's a side effect of Japanese heroic storytelling.
Most heroic stories in Japan revolve less around the hero as a person but the hero's tool or weapon making the hero. "The Holy Sword chooses the hero." Sounds familiar? "Whoever wields *insert weapon name* is invincible!" Or something along those lines. The weapon ends up making the hero and not simply the hero being heroic. There are similarities to European storytelling, most notably Excalibur from the Lady of the Lake. There may have been cross influence at the time of Japan's contact with Europe.
Additionally, the weapon or tool ends up being the identity of the hero. They're more often identified as "wielder of X". The weapon or tool is now a calling card, a signature of the hero.
In an isekai, this serves to identify the MC as unmistakably Japanese. Especially with isekais that have the MC into a different body than their original Japanese one; they crave for Japanese soy sauce, miso, Mirin, etc, they crave Japanese habits such as hot baths and shouting "TAMAYA" during summer fireworks. But in a work where that may not make sense, the katana ends up identifying the work ITSELF as a Japanese made work.
Compare and contrast with say US heroic storytelling, where the hero is the hero because he has heroic qualities. The weapons and tools don't make the hero, the hero's character is what makes him a hero. War stories focusing on the grunt, on firefighters and good cops surrounded by corruption. The hero doesn't isn't chosen by weapons or tools, though they may be identified with it. Neither are they solely identified by their tool.
Also compare and contrast with Chinese heroic storytelling, where similar to the US example, heroes have a high level of ability and skills to identify them; old example being Crouching Tiger, Hidden Dragon; one of the heroes is able to beat the current wielder of the "invincible" Jade Destiny sword....with a stick.
