Which actually works, because that's exactly what you'd have to do!
Where heavy petroleum fractions and other applicable refinery streams are broken down into lower molecular weight hydrocarbons
The cat feedstock is some nasty material that looks worse than most crude oil.
It would seem feasible to introduce a small percentage of waste hydrocarbon plastics to the cat feed as it was being processed, in order to at least recover a bit of the fuel value from the plastic.
I can only imagine it is simply cheaper to continue to purchase the equal number of barrels of regular cat feed than it is to attempt to displace some of it using waste polyethylene & polypropylene which may even be possible to deliver at no material cost.
OTOH, for all I know people may be doing this already.