Complete the Collection

The urge to collect all the items, achievements or secrets in a game.

The desire to collect things is a strong urge that game developers often exploit to get you to keep playing a game with the hope that one day you'll find all the rare items and have a complete set.

For people who collect things, the collection can have significant emotional value. It may connect them to something that they enjoy or feel strongly about. Some people collect for the thrill of the hunt, others may collect because it gives them a sense of security or control. Others may derive great satisfaction from having a complete set and feel anxious when they are missing a piece of a set.

All of these psychological effects contribute to making you keep playing the game, even if the "playing" of the game is not enjoyable anymore. If game developers are constantly releasing new content, then the players are always needing to "catch up" and may feel like they need to spend money to do so.

Sometimes reaching the 100% completion rating in a game can take hundreds of hours of additional tedious work beyond the normal scope of the game. Sometimes, some of the collectable items are only available via special events, and because players may have a fear of missing out on those items, they will continue playing and paying, even if the game is no longer fun.
Citations and SourcesWikipedia :: Psychology of Collecting


Examples

Cat Game - The Cats Collector! Cat Game - The Cats Collector!
"Vaguely yes, there’s mentions of collecting them all and the goal is simply to collect."

World of Tanks Blitz MMO World of Tanks Blitz MMO
"Badges, tanks up to tier 10, avatars, ..."

Walking Dead: Road to Survival Walking Dead: Road to Survival
"There is a museum that when you get certain items you can complete for more items or characters."

Magic Duels Magic Duels
"Rarer cards tend to show up more frequently when you near the end of collecting a set."

LEGO® Star Wars™: TCS LEGO® Star Wars™: TCS
"Not really dark though."

Crossy Road Crossy Road
"There are hundreds of characters that are obtained either through a random draw or through real money."

Ridiculous Fishing - A Tale of Redemption Ridiculous Fishing - A Tale of Redemption
"Collect all fish-"

Hearthstone Hearthstone
"I mean, the game doesn't "urge" you to do this, but if you want to be competitive, it certainly is helpful to have all the cards. Although again, it is far from necessary. And the game doesn't actively force it down your throat or anything."

Animal Crossing: Pocket Camp Animal Crossing: Pocket Camp
"There is a catalog, which shows a crown icon if you collect every item in a set."

Homescapes Homescapes
"In this game you are trying to fix up your house."