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

The Sims™ Mobile The Sims™ Mobile
"Furniture and clothes collections."

ANOTHER EDEN ANOTHER EDEN
"The aforementioned award system has awards for completing collections of weapons, materials, characters, fish, etc."

Mech Arena: Robot Showdown Mech Arena: Robot Showdown
"You can collect skins for mechs and mechs themselves"

Flip Tumbling Flip Tumbling
"Get new characters"

Make More! Make More!
"There's a collection of workers & items. The game tells you which workers are new; and every 100 items you get +1 tapping power"

Castle Cats:  Idle Hero RPG Castle Cats: Idle Hero RPG
"It the whole point of the game."

SCRABBLE SCRABBLE
"Yes, different tile sets which are simple visual and offer no extra power or experience."

Infinite Lagrange Infinite Lagrange
"There is a set of ship blueprints to collect"

My Singing Monsters My Singing Monsters
"monsterpedia, there are different versions of the same monster too"

Pokémon GO Pokémon GO
"Catch 'em all!"