Sort of like your settlement’s new hearthlings “magically” appear on the terrain near your settlement?
Look, this isn’t a kingdom simulator. Goblins represent danger, and having repeatedly to deal with the goblins represents ongoing danger. Who cares how the game spawns them?
If you’re desperate for an in-game explanation, try this: the goblin chieftain, having lost the armsmen he sent after you, sent a message to Central Goblin Armsman Dispatch Service asking for reinforcements, which were sent a couple at a time (available reinforcements are scarce, as goblin chieftains all over keep losing armsmen and demanding more, and breeding and training take time); the goblin chieftain, being a dumb goblin, keeps throwing them at you as soon as he receives them from CGADS; CGADS Chief Administrator Muktuk is getting tired of this, but his hands are tied as he has strict regulations requiring him to fulfill all reinforcement requests as soon as possible, so he sends a couple every day.
So can you please just play the game, now? I’m not understanding how this is unrealistic, when we’re talking about fighting goblins with wooden swords.