There would be serious logistical issues with this. Drive by the nearest refinery and see the cracking tower. Essentially this is doing as you describe, making ethylene, propylene etc from olefins. But these refineries are located at pretty centralized petroleum or gas terminals. Compared to the mass flow of used plastics distributed evenly across the population density. You either need the haul the stuff very long distances or build lots of huge refineries everywhere. An incinerator is a significantly smaller capital investment than a refinery.
Cracking actually doesn't need tall towers. If you see something tall on a chemical plant, then you're likely looking at fractionating columns, not the cracking reactors.
I happened to do a project on their fluid dynamics at university :)
They don't have to be this tall, if you need lower throughput or if your products don't have wildly different boiling points.
How cheaply, in theory, could you mass produce cracking plants for post consumer waste?
And the remaining 10% can be burned, of course.