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

WWE SuperCard – Multiplayer Card Battle Game WWE SuperCard – Multiplayer Card Battle Game
"It's impossible to 100% the catalog because of how early events worked, but there are small collections that were introduced this season. Collect certain cards in a tier to get a random card from the one above."

Legend of the phoenix Legend of the phoenix
"Outfits often have low value items like shoes or socks that give a bonus for 100% completion You're encouraged to collect "precious collection cards" You're encouraged to complete confidant stories You're encouraged to "collect them all" when it comes to partners, including expensive Duke or Prince Tokens for recruiting high-value partners"

FINAL FANTASY Record Keeper FINAL FANTASY Record Keeper
"Characters get better by obtaining all possible items related to them"

Girls' Frontline Girls' Frontline
"There are bonuses for completing furniture sets and achievements for collecting certain numbers of characters or other milestones."

ISEKAI:Demon Waifu ISEKAI:Demon Waifu
"Collectible waifus"

Candy Crush Saga Candy Crush Saga
"You get a crown for completing every stage in a chapter on the first try"

Off The Road - OTR Open World Driving Off The Road - OTR Open World Driving
"There are 4 different things you can collect on each map. Flags, Cards, card packs and challanges"

#DRIVE #DRIVE
"The game is essentially centered around collecting every vehicle, "Cool Version", and map. A good chunk of this can be skipped with the $10 purchase."

LEGO® Legacy: Heroes Unboxed LEGO® Legacy: Heroes Unboxed
"You are encouraged to complete every level, every event, and collect every character."

GrandChase GrandChase
"A small amount of gems (premium currency) is given for each hero per evolution level and each piece of equip when you obtain them for the first time. Gems are also given when you complete certain collections of heroes in the game."