Is this partly due to business practices and emphasis on agility? Where everyone is afraid of investing time documenting systems that are possibly (even though often times unlikely) to change in the near future and a fear of sinking resources into increasing efforts (original documenting effort + modifying documentation effort in future if changes occur)?

On top of the initial investment in documenting up front, all too often when future changes come about the documentation is neglected (often due to pressure form the business, or forgetfulness) in the estimated amount of work to achieve a task. Incorrect documentation is often more costly than no documentation.