Tallow is more expensive, so if it were the only option, fried food would be less profitable. And that would mean there would be less fried food in our diets.
There are some recyclers now that will pay you a bit for used oil, but I doubt it's enough to make it cheaper to buy fresh vegetable oil rather than cooking the fries in the beef fat you already have.
then again maybe its cause those taste the best due to being evolutationary selected and companies arent trying to kill us but get us to buy whatever our bodies easily go for.