The perception that ninja swords are straight is a common misconception. In reality, historical evidence and records suggest that ninja, also known as shinobi, did not have a distinct type of sword that was unique to their profession. Instead, they used a variety of weapons depending on the situation, including swords, daggers, and various improvised tools.

The portrayal of straight swords being associated with ninjas in popular culture may stem from the influence of fictional depictions, movies, and comic books. The straight sword, often depicted as a ninjato or shinobi-gatana, is a fictionalized creation that has become associated with ninja characters.

It's important to note that historical accuracy regarding the specific weapons used by ninjas is challenging to determine due to the secretive nature of their practices and limited historical records. The primary focus of ninjas was on espionage, intelligence gathering, and stealth, rather than being identified by a particular type of weapon.

In summary, the idea of ninja swords being straight is largely a fictional construct and not reflective of historical reality.