mobs don't pick up items

Not all mobs can pick up items. Depending on your difficulty only a random fraction of mobs will be able to pick items up.

The mob's "picking items up" ability is determined by a TAGByte {CanPickUpItems:<1/0>b}.
As said above, only a small fraction of them can pick up items, wich only applies to:

• Zombies (all types)

• Skeletons (all types)
  If you want to summon a zombie/skeleton with the ability to pick up items, use this command:

  /summon <zombie/skeleton> <x> <y> <z> {CanPickUpItems:1b}