That is a signature speech style of samurai and ninja depicted in modern fiction. They also typically speak like , which is a plain form of . I think there are several reasons they speak like this.
* Stereotypical samurai are loyal, but reticent, proud and dignified at the same time. They don't have to be very polite like a servant or a butler.
* as a politeness marker derived several hundreds years ago as a contraction of (see this). derived from . Therefore, / was not the most polite style in the age of samurai. In samurai dramas, we hear peasants and merchants say / often, but people with high social status seldom use them, especially when they speak formally.
* Last but not least, if that was said before a battle, who wants to speak politely in a battlefield?
This type of is used also by a proud Western knight before a duel. is almost specific to samurai/ninja.