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

Dead by Daylight Mobile Dead by Daylight Mobile
"The game has a page where all cosmetics are displayed for each character, showing blank spaces and a counter of missing parts of the "complete collection"."

Single City: Life Metaverse Single City: Life Metaverse
"avatars, roomies(characters), limited furniture"

The Walking Dead: Survivors The Walking Dead: Survivors
"Collect and build to earn achievement and reward"

Temple Run Temple Run
"There are collectible characters"

Solitaire Cash - Real Money Solitaire Cash - Real Money
"Seasons of card collections, large reward if you get all of them- but you get more and better cards by gambling more money."

Polysphere Polysphere
"Some of the puzzles are paywalled, so even if you want them all you can’t without money"

Life Makeover Life Makeover
"This is a clothes collection game"

Tap Away Tap Away
"Again, SO MANY. Puzzle pieces, skins, blah blah blah it has nothing to do with the game and there's no opt out or side menu- it's shoved in your face involuntary"

Night in the Woods Night in the Woods
"Mildly, it has concept art that you only get if you play the extra mini games or if you get to a certain part of storyline- it doesn't tell you in game when you get these though, and it's not shoved in your face, but rather in a separate menu on the home screen"

Limbus Company Limbus Company
"often people will pull only to "have everything" or complete sets of characters"