Why don't FMCs automatically include up-to-date oceanic route definitions?
Much like 'Why can&#39;t flight management systems receive instructions directly from the controller?', the answer is it should be coming soon (mid/late 2020's).




  
    FMCs are pretty clever, and they can (and regularly do) pull down data from the network.
  


You're right, but the issue is "the network". They do that already off private networks, as opposed to a common public network (Aeronautical Telecommunication Network). ATN is now at the heart of the data-link modernization.

Having the option to access and uplink common flight plans and routes to an aircraft's FMS should become a reality (Boeing):



But as the above 2016 presentation shows (starting from the 'B2 Ground Deployment' slide), some old hurdles are still there. A 1994 Flight International article might shed some light on why it's been taking so long:


  It is increasingly clear is that different regions, operators and manufacturers are going to find their way to the ATN by different routes via different steps, and that is giving rise to a variety of concerns. The most avid proponents of the ATN fear that some entities will become content with incremental communications improvements on the way and lose the desire to see the project through.
  
  Airlines are reluctant to make the necessary investment until civil-aviation authorities (CAAs) make their intentions clear. In a classic chicken-and-egg paradox, however, the CAAs say that they cannot go ahead until they are certain which equipment the aircraft will carry in future. Exasperated service providers are reduced to pleading with the user community to do something, at least.


ATN has been a dream since at least the early 1990's, but reluctance is the reason. But whether the route is oceanic or not, the pilots are supposed to check the FMS entries whether manually typed in or uplinked.