Monsters Everywhere

In normal stories, heroes often encounter enemies. They may be found guarding some important MacGuffin or perhaps sent to slow the heroes' progress. They are spread out in a few locations where they make sense. In video games however, and RPGs in particular, going from A to B is like carving your way through a thick jungle of flesh.

This is because there are simply Monsters Everywhere. There are monsters on the road connecting two cozy peasant villages, in the carrot fields, behind every tree in the forest, under water, in the air and in space. That abandoned tower? Full of monsters. Every single floor. The mine being excavated by poor workers? Inhabited by monsters that attack everything except poor mine workers, because after all everything is trying to kill you, not anyone else.

It doesn't even matter if the The Evil Overlord needs you to make your way to him to do a Hostage for Macguffin exchange—his henchmen will spawn and brood like cancer everywhere the hero goes, because monsters are a force of nature in the RPG world on par with grass and trees and the existence of air.

This whole thing is obviously for gameplay reasons, to whit, there wouldn't be any otherwise.

You will not find any monsters in small villages however. This is due to an existential dilemma in which monsters need people to terrorize (by standing right outside, roaring).

This trope exists mainly because RPGs Equal Combat.

Examples of Monsters Everywhere include:

Video Games

All RPGs, especially older ones and MMOs.
The earlier Final Fantasy games eschew Chest Monsters in favor of having giant enemies somehow living in the boxes.
Nethack. Horses, bees, trolls, elves, snakes, demons - and everything in between - grows out of rock. Or perhaps they are spawned by the evil Wizard. But why then does he spawn a puny rat to defeat the hero that just killed five dragons without breaking a sweat? Maybe to maintain a certain ambiance? Kitten and Vampire Lord fights side by side!
Sacred 2 Fallen angel has monsters and bandits on every road, in every forest, and right outside every village. Exacerbated by them respawning almost as quickly as you can kill them. One wonders how any settlements got built in the first place, let alone how anybody manages to travel anywhere with the apparent apocalyptic horde hounding anybody who ventures outside the cities. Kind of gets old when your LV 25 mage is still being attacked by endless waves of rats and kobolds.
Pokémon, especially in caves. It's even worse in HeartGold and SoulSilver, where more appear if you run, because the noise attracts Pokemon.
Handwaved by the .hack MMO The World and The World R:2 - there's just so many monsters that civilization has condensed itself into cities protected by energy fields or distance ("Root Towns"). Access to this vast wilderness of monsters ("fields") is provided via Chaos Gates which, supposedly, only the players can use. One wonders why it is even necessary since, in a typical moment of Gameplay and Story Segregation, players need to first approach enemy spawn points on fields before either can attack the other.
Standard in Diablo 2, where anywhere other than the towns will be absolutely swarming with monsters, including the sewers under the town.
Golden Sun: Dark Dawn lampshades this during the Grave Eclipse where stronger monsters start appearing. Your party has to escape to a boat and when they arrive, one of your party members will say how they couldn't go five steps without being attacked by monsters.
Tales of Vesperia actually explains in the backstory that the world is overrun with monsters and most people never leave their hometown because of it. Towns are protected by giant Deflector Shields.
Justified in several of the Shin Megami Tensei games because the world is experiencing an apocalypse or its immediate fallout caused by the sudden emergence of demons into the human world. Specific sub-series follow or occasionally avert this trope with their own justifications; Persona 3 and Persona 4 are the most widely known aversions, confining enemy encounters to magical otherworlds where the monsters exist naturally, allowing events in the normal world to carry on largely without issue. Devil Survivor also averts this by confining encounters to either specific plot-important fights, or "free" battles found only at specific locations to help grind and prepare for plot fights.
A staple of The Elder Scrolls series. If it's not monsters, then it is bandits, and typically every single roadside fortress that is not a spitting distance from a town is overtaken by them.
Paragon City is supposed to be the eponymous City of Heroes, but at any time one can easily find crowds of gang members committing arson or mugging bystanders right across the street from the City Hall and the place most likely to be hosting several dozen superheroes at that very moment. Especially bold criminals would be committing crimes in the City Hall parking lot or at the base of the staircases leading up to the City Hall and the statue of Atlas.
- The Rogue Islands from City of Villains are much the same, but it makes more sense, as they are a corrupt nation run by a villain group which espouses a Social Darwinist agenda and encourages rampant crime in the streets as well as heavy-handed "police" tactics.