These take time to draft properly, and to make sure that they comply with the parties intentions. Disclosure of assets, debts and income is key, as it allowing sufficient time to seek independent legal advice. Otherwise, the agreement may not be enforceable many years later.
Additionally, it is possible to have a perfectly valid agreement, but to have it be rendered completely useless by the parties actions afterward. It is important to know what not to do in the years following the agreement.