Because our perception of ninja is based around kabuki theater and not historical accuracy. Kabuki theater stage hands were always dressed in black to blend into the background. When an ninja assassination would happen in a show, the "stage hand" would do the assassination because the audience knows to kinda ignore the stage hands during the production.

IRL ninja would just look like normal people, waiting for the right opportunity to strike at someone.

I'd go with all the above from both comments.

Yup, and it makes perfectly intuitive sense once you know about it.
If you've ever used a software product with one of those left hand menus with a big list of capabilities from any "big" company, it's almost assured that each item in that list is it's own development team that's only tangentially aware of what the others are doing, and the team in charge of maintaining the menu.

I was on a team for a bit whose goal was to find places where we were shipping our org chart and make our tools play more nicely with each other.
End result: we found some really good areas to make them play better with each other, implemented them, and... They got their own entry in the left hand menu because maintaining a feature fully integrated with four disparate teams with different goals is really hard.
To our credit though, once you turn it on, our thing makes the lines between the products essentially disappear for our end users.

You have to be mormon...they hire from BYU