Some of these things go back to the earliest text adventure games. Colossal Cave spawned countless imitators during the time when text adventures roamed the Earth including the still popular Zork games. Text adventures morphed into graphical text adventures, point and click RPG's and then 3D, first person perspective RPGS and MMORPG's and some of the elements of those early text based games were retained at each step along the way most likely to retain some feel of familiarity for the gamer who may remember some of the earlier games that came before.

Malik wrote:

And, oh, the rats in RPG games. They seem to be the welcoming committee members in the RPGs.

And snakes. Don't forget snakes. Snakes and rats seem to be interchangeable as pests for noobs to kill to level up their skills. And also don't forget the incredibly huge boss monster or sinister, super-powered villain at the end of each level or at the end of the game that needs to be killed to advance the storyline.

Underground caverns filled with goblins, orcs, gnolls, kobolds or other sentient species in need of genocide is also an overdone idiom in RPG games. The same is true of quests involving ridding the land of wandering gangs of bandits who waylay innocent travelers and relieve them of their worldly goods. I blame Dungeons and Dragons and it's clones for these as many of the game modules have either the main, or at least one side quest, or as a possible random encounter, something involving these types of antagonists.