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

#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."

Dragon Mania Legends Dragon Mania Legends
"The main goal of the game is to breed and collect as many dragons as possible."

Mobile Legends: Bang Bang Mobile Legends: Bang Bang
"Achievements"

Dream Girlfriend Dream Girlfriend
"Not really, but some dresses only fit certain accesories, which are near impossible to match on another outfit. It feels like having one shoe instead of a pair."

Lovestruck Choose Your Romance Lovestruck Choose Your Romance
"Puzzle pieces collection, some of them limited time for collection"

Arknights Arknights
"Recruitment-exclusiv​e Operators are especially hard to get, which keeps even veteran players logging in to start those timers."

AFK Arena AFK Arena
"Dupes are the main way to power up heroes, so you end up collecting them all and seeking out as many copies as possible."

Epic Seven Epic Seven
"Only for competitive purposes or unlocking side stories."