Leather is not a byproduct. Virtually no one is raising cows just to be nice. It's a business, and no business is going to waste resources on unprofitable "assets". This means the cows are raised specifically for their skin (in the case of the leather industry), and they are killed while still young. The same is true for both the animal flesh and dairy industries - older cows are less profitable.

So to accurately compare it to humans, imagine a bunch of babies and young children being confined in cramped, unsanitary conditions, regularly getting abused, and then being slaughtered long before they ever had any opportunity to do anything with their lives - having only ever known suffering and abuse.

And you think there's anything normal about that?