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

Mario Kart Tour Mario Kart Tour
"Challenges. Challenges come in sets of 9, in a 3×3 layout. If you complete a challenge, you get a small reward, but if you complete a row, column or diagonal of challenges, you get an additional reward, and if you complete all nine, you get another reward: a badge, a ticket, some rubies. And in races, you can earn 500"

Harry Potter: Wizards Unite Harry Potter: Wizards Unite
"There are a limited number of Foundables. Some very very rare. The point of the game is to collect them all. Not sure if it's actually possible."

Star Trek Fleet Command Star Trek Fleet Command
"Although it would be absolutely impossible."

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-"