Leather seems an obvoius candidate for making armour, it is cheap, plentiful and can be made pretty tough.
Sheilds were often coated with leather. A collector friend of mine has a 19th century sheild from Africa, made of hippo hide. It is amazingly tough and heavy. Apparently the original owner though it could stop a (19C) bullet. I can see why this might be tought to be true. Zulu sheilds are made from animal hide, although they look relatively flimsy.
Leather was a common component of armour, being used to mount scales, connect parts of articulated armour or as a padded undergarment etc. And, of course, leather was used as a type of armour in later years - I am thinking of those heavy buff coats worn in the English Civil war etc.
However, I have not seen any convincing evidence for early armour made of leather - where it is the leather that provides the protection.
Is there any direct evidence from archaeology or literature?
What is the earliest use of leather armour?
If leather was not used as an armour, why not?
There are descriptions of leather shields in The Iliad, IIRC. I don’t recall any mentions of leather armor, offhand.
If you read Bernal Diaz del Castillo’s Conquest of New Spain, you’ll find descriptions of the Cotton armor used by the Americans. The Spaniards came to prefer it, too – it was a lot lighter, and you could sweat through it.