All journey pairs have fares set in the database, it’s just that illogical ones aren’t displayed on fare finders. In many cases the set fare is correct, though I have found some which are wildly inacurate.
I’m in agreement that they really ought to show fares, but accept that some of these journeys are unlikely to be made. Another example is Thameslink fares from Luton AP do not include stations on the Edgware branch of the Northern line because no-one in their right mind is going to change at Kentish Town* and Camden Town to head back to Hendon Central, when they could get a bus from Hendon.
*obviously even less likely while the Northern line isn’t stopping.