Usually this protects you from machines and machines are rarely sued or sue. Network documentation is difficult to do yourself, since it often relies on work performed and knowledge possessed by a vendor. The vendor is focused on getting and keeping the technology working. In fact, documentation may allow you to be more independent of the vendor. Remember that not all vendors encourage customer self-sufficiency with technology.
Network documentation has a very short shelf-life.
Most network environments are changing frequently. Network documentation is too time-consuming. Network documentation like all other documentation, takes time. The value of the time invested is not justified by the time and cost it saves in the instances that it is relied on. Finally, you need to maintain some autonomy from your vendors or at least know who to call.