Well no, mostly we don't, because the brains who happen to be the weapons still need a world to live in after they fight. The difference is that with artificial brains, hey, why would they give a crap what comes after the killing?

However, we mostly only arm brains that have an IQ of <120 and some human values baked in (e.g. empathy to other humans and living beings; a family to protect, or dependence on other social structures that demand pro-social attitudes).