"There's no sensible reason for using animal fat when making money,"
You don't actually thing there's a special farm somewhere fattening up animals for the slaughter just to make fivers and throwing away all that meaty and leathery by-product, do you?
If the fat is being used to make some plastics, it's most likely that it's "waste" product. Selling it for other uses instead of disposing of it (at a cost) means less waste so those of us who have chosen the blood thirsty, murdering lifestyle of eating meat can feel better that more of the animal is useful in death.