game design - Why have the player pick up loot manually?

From a game-design perspective, I was wondering what the mechanic of manually picking up items off the ground does for the player. Keep in mind that items that fall on the ground can most likely be put into one of two groups:

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  Equipment Items: depending on the character the player is creating, the majority of these items might be ignored and left on the ground. Players will only pick up the ones that they want to "spend" inventory space and time (by clicking and eventually having to go back to town) on.

  
  


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  Consumables: gold, gems, crafting materials, etc - which players generally always want to pick up and sometimes don't take up inventory space.

  
  

While I can see that making players choose which items to pick up is an interesting "collecting" mechanic for the Equipment group, why make the "Consumable" group require manual picking up as well?

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Examples of games which do this: (most diablo-esque dungeon crawlers)

Diablo 3

Path of Exile

Grim Dawn

Torchlight 2

Answer

Why are you giving the player loot in the first place?

It's a reward stimulus for the player. Games need a constant stream of reward stimulus (or at least the anticipation of a reward) to keep the player motivated to continue playing. The impact of a reward is not just determined by the mechanical effect (player now has 10 more gold to spend) but also by how much the player notices it.

By having the player interact with the reward in form of clicking to pick it up, you have the player notice it more which amplifies its psychological effect without changing its mechanical effect.

It also makes the player more aware of their current possessions. When you just put stuff into their inventory directly, they might easily miss that they just had the luck of finding the ultra-rare +99 sword of pwnage because they were focusing on something else. Then half an hour later they open their inventory for a completely different reason and wonder how they got it. The same applies to more generic resource-style items which the player collects in bulk. By getting a more direct feedback about the loot reward they get a clearer impression of which actions reward them with what kind of loot.