In modern FMCs, flying an oceanic route requires inputting coordinates directly, often in obtuse formats (which are arguably prone to typos, but following proper procedure and a step through the route map generally prevent this being an issue).
Working with an example: North Atlantic Tracks, or NATs, are a set of roughly parallel airways in the North Atlantic, generally spanning from Canada's maritime provinces in the West to the Irish coast in the East. Because of frequently changing conditions in the North Atlantic, these routes undergo changes every few days, which are conveyed to pilots before each transatlantic flight.
Having said all that: modern FMCs are pretty clever, and they can (and regularly do) pull down data from the network, including in-flight weather data, for example. Why don't modern flight computers use this to also download oceanic route definitions if manual input is required rather than using a company route?