It’s not just the cost of disposal. Recycling aluminum or steel works because the alloys can be separated into their constituent parts and recombined into an alloy of the same or higher quality. That recycling them is cheaper than extracting it from ore is just a nice bonus that obviates the need for political incentives.

Recycling plastics breaks down the polymers into smaller ones which degrades the quality in almost every respect. The cheapest way, for example, is literally just shred the plastic into lower quality injection moulding pellets.