Written constitution reflect the un-Written social contracts, already in place at the time of the writing. Because times change, if the written contract is too inflexible, the constitutions will fail. If it is too easy to change (amend), it will be subject to the popular whims and fears of the times.
The US Constitution, for example, does not mention a god any place in it. It was a lucky guess, because if it had, the US might still have laws on how to identify witches and warlocks, and how to punish, and kill them.