There have been many decentralised legal systems. In fact it was the norm for most of humanity's existence, persists in "ungoverned spaces" not yet oppressed, and springs up again whenever the boot of monopoly fiat "justice" is removed from a community's neck.
https://mises.org/library/enterprise-customary-law
https://mises.org/library/enterprise-law-justice-without-state-0
It's still the norm, to some extent. Plenty of "official laws" stop being prosecuted and eventually are removed from the books. And public outrage can lead to the creation of new laws.
Legislators try to "get in front of" the organic law definition, so that they can lead the specific definition of the law.
Common Law is a very interesting "living fossil" - a hybrid of an ancient precedent-based traditional legal system and a "modern", "fiat", politics and legislation-based legal system.
Nowhere near enough outdated "fiat" laws are removed from the books - there are no photo ops or announcables to be had in doing so; and their accumulation is a serious problem when it comes to the accessibility and equity of the legal process.
If you like law, you should read Szabo’s unenumerated.blogspot.com
