The following are just my thoughts on why an Airship could be important to very late game, the games ending and progression into a part2:
Part2 might have extensive travel/adventures. Radiation zones could be 100's of miles across, you might miss a river or a settlement if you are driving and struggle with impassable terrain. Small planes don't have the range.
The whole process of getting the large amount of resources to make one including finding the plans for one would take the last 1-3 weeks of the game and escape with one other NPC before some disaster happens (for a good climactic end).
Part 2 might have many maps that need to be explored and an airship would have the endurance.