Yes, humans cannot eat grass (for example), but grass is also not a byproduct. And fields used to grow grass could be used either for other (human-edible) crops, habitation or for wildlife restoration.

It probably depends on the source, but here (france) you can ask your butcher for byproducts to feed to your cats/dogs and they give it for free.

That is incorrect. Around 40% of fields are used to grow food for livestock, as well as a quarter of fish caught being fed to animals. I found this article saying we could slightly increase byproduct use for feed, but they are minor improvements. https://www.nature.com/articles/s43016-022-00589-6