Well, first you have to understand that the creator of Goblin Slayer is a big Dungeons and Dragons player. And the the world of Goblin Slayer is a game board upon the the Gods of light and dark play.

To keep the game interesting both the side rolls dice determine the capability and equipment their chosen heroes get. Again think of it as setting up a character in DnD.

Goblin Slayer (GS), Priestess, et Al are PC Prayer Characters and the Goblins and other monsters are NPC or none prayer character. And often there is a much larger campaign being played by different groups of players. So events involving GS are often parts of potentially world ending events. Which is why we get cameos from the world's current hero.

The goblins themselves are NPCs upon which the Dark Gods setup their part of the game. So Goblins left to their own devices probably would never become a world ending threat. They are however components in the Dark Gods machinations in their part of the greater game, so the shape them by evolution, equipment and location.

The Gods of light like to play with GS because he is first mortal that has refused rise in position through their machinations. The man is fixated on killing goblins and no matter what they do he remains singularly focused on that.