Because the basic crimes are so universally understood and detested, that there needs to be nothing written. Murder, theft, robbery, etc. Every person knows from birth that those things are wrong, and it takes severe brain washing for people to change their minds on it.
> Murder, theft, robbery
Have inconsistent definitions over time. Hence my example of legalised murder in post-Norman conquest England, and cultures without personal property where theft is a nonsensical concept.
Also, just ask around left- and right-coded answers to "is taxation theft?", or in the US specifically "is abortion murder?" or "is the death penalty just state-sanctioned murder?"
And, pertinently to this thread, when is a military action murder vs. not murder? There's lawyers looking at the videos of Venezuelan boats being destroyed saying "the US murdered those people". Is one of the sides here, pro or con it doesn't matter which, a victim of "severe brain washing"?Similar disagreements (albeit by non-lawyers) are had regarding all wars I recall in my lifetime.