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

RuneScape Mobile RuneScape Mobile
"A bit. Oh, hello, quest cape, max cape, master quest cape, master max cape, completionist cape, trimmed completionist cape..."

Cats&Soup Cats&Soup
"Game encourages you to collect cats."

Kuma Sushi Bar Kuma Sushi Bar
"There is a catalog for the customers you have unlocked/collected."

Zombie Frontier 3: Sniper FPS Zombie Frontier 3: Sniper FPS
"Clearing achievements also gives a reward (coin,gem,item,weapo​n skin,and also Play Games XP) further encouraging players to play more to unlock all of them"

Vampire Survivors Vampire Survivors
"Clearing achievements also unlocks items, characters, and upgrades, further encouraging players to play more to unlock all of them."

Mystic Messenger Mystic Messenger
"Multiple endings, CG gallery, party guest gallery"

Million Onion Hotel Million Onion Hotel
"Unlocking every achievement is necessary in order to view the game's true ending."

Little Alchemy Little Alchemy
"collecting them all is both an achievement and your end goal with the game."

Nameless Cat Nameless Cat
"achievements for collecting certain numbers of fish cans (1 is in every level) and postcards (3 total in game)"

Papa's Mocharia To Go! Papa's Mocharia To Go!
"The more achievements you complete, the more stickers you get which can unlock new costumes for customers. Some achievements are unlocked just by progressing in the game so you do have to continue playing to get them but you aren't forced/urged to complete any achievements to advance the game."