The fun of collecting bigger and better items in a video game is self-evident to many players. Picking up a new gun which inflicts four times the damage of the one in your character's hands -- in addition to an array of interesting modifiers like corrosive or electrical damage -- easily puts a smile on my face.
Finding new loot is like a mini-reward for adventuring. As players progress further into a game and as quests become either tedious or monotonous, the chance of obtaining better loot can sometimes be the only driving force behind player progression. Loot's allure is that powerful.