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

Isekai:Slow Life Isekai:Slow Life
"Yes, you get small bonuses."

Wuthering Waves Wuthering Waves
"Being a completionist directly contributes to how much freedom you have to building characters"

AFK Journey AFK Journey
"World map has completion bars for each area that offer rewards. Events and limited time cosmetics for the player character / heroes."

My Talking Tom Friends My Talking Tom Friends
"pieces for albums you earn from card packs you earn from loot bags (see my review on loot boxes)"

TAP! DIG! MY MUSEUM! TAP! DIG! MY MUSEUM!
"kind of the point of the game"

Talking Tom Hero Dash Talking Tom Hero Dash
"armor and characters you can play as"

Forge of Empires Forge of Empires
"during events, they have many sets to complete. Some are just standalone buildings that can be upgraded and others are things you need to assemble together to create a set of something."

Beatstar - Touch Your Music Beatstar - Touch Your Music
"Getting all of the free songs in the base game unlocks Unlimited Play until they add more content. This is essentially a 100% collection award, providing something to work towards. However, they made this goal nearly unobtainable for casual players with the Deluxe update. (Deluxe songs take more than double, even quadruple the time of a normal song)"

BitLife - Life Simulator BitLife - Life Simulator
"heirlooms, challenges, accessories, artifacts, careers, achievements, treasures, designer clothes, ribbons, scavenger hunts"

MONOPOLY GO! MONOPOLY GO!
"Stickers, which you can also buy with money. Has special golden sticker that are required to complete the collection but you can't trade them with friends, only randomly earn or buy loot boxes that may contain them."