Oh no, here we go again with the "we use leather armor because metal is too heavy" trope.
Yeah, it really existed for a while, and was quickly replaced without looking back when plate armor was invented.
To offer any protection, leather armor had to be hardened by boiling it, removing any flexibility advantage it could have over metal; layered, removing weight advantage over metal which only needed a thin sheet to offer the same resistance; and required much more care to not dry and crack.
And if it's a fantasy world with mythril or other mythical material, that's even stupider. That'd be like building a bicycle out of wood because "iron's too heavy" when you have access to aluminium or carbon fiber.
Someone should write an isekai where the MC behave according to these tropes, and everyone treats them as a weirdo because none makes any sense in the "real world".
MC: "I want a leather armor!"
Armorer:"A what now? Why'd you want that?"
MC: "I want to remain fast and agile!"
A: "Errr... Whatever, it's your money. I think I've got one my great-great-great-grandfather made as an experiment"
MC, having donned his leather armor, later get thoroughly crushed in battle against a lower-level knight in full steel armor that's as fast and agile as them, but can withstand much heavier strikes without damages.
