When I worked at a plastics blow-molding factory a decade or more ago, the main products, clear plastic ketchup and BBQ sauce bottles which they made en-mass, were limited to only like 1.7% of recycled material.

This recycled material was the pure, no sorting needed, ground up waste material. The limitation was due to the plastic quality. It must meet requirements of elasticity and strength; not too brittle to crack versus not dented too easily.

I'm not sure what made the fresh waste material mostly unusable, but it was likely from the heating and cooling process and additives. Additives I'm aware of include UV stabilizers, so bottles don't decompose if left in direct sunlight very long, and "lubi-stat", as light food-grade oil that prevents the plastic from getting scuffed up against other bottles.