Mobs/animals drop all items

I think that in Minecraft mobs and animals should drop all items. For instance, if you kill a cow, it will always drop 1-3 pieces of meat and at least 1-2 pieces of leather. If I kill a skeleton, it should drop a bone, bow, and arrows. Another example is if I were going to kill a zombie that's in full gold and I kill him, he needs to drop all the armor even if the has low state. Have you ever been trying to get leather armor and you come across a few cows and like none of them drop any leather? It is frustrating! This is why I think that animals and mobs should drop all items.