But another perspective is that you're recycling petroleum. That is, we make plastic polymers:
petroleum -> chemicals -> plastic polymers
old plastic polymers -> chemicals -> new plastic polymersTake for example polyethylene, a product we can recycle relatively well. The first reaction gives polyethylene chains of length N. Because they're as long as a polyethylene chain can be, they're as strong as polyethylene can be, and have the best performance characteristics you can expect. Each time they're heated up to be remolded or have contaminants removed, and to a lesser extent just from use and contact with sunlight, some of those chains break. This creates more N/(2^x) length chains. This causes the plastic to get weaker over time. The energy cost to recombine those chains would consume more petroleum than simply creating new plastics, if it could be done at all.
In a hypothetical future with 100% cheap renewable energy, we could recycle plastic over and over?
As an imperfect analogy, we used to mine fertilizers. Now we use energy to create ammonia directly from nitrogen in the air.
In a closed system, where you're just cracking plastics, it's arguably easier to deal with contaminants. And I also suspect that you'd use less energy overall.
Sure, you could theoretically break down most plastics to CO2 and the like. They're a lower energy state than most plastics. The question is why expend the energy to get something we have a huge surplus of. You're not going to be able to get volatiles such as methane or ethylene out of that process efficiently, because it's a higher energy state than the plastics. Tar sands and the like are a mixture of volatiles and heavier compounds; they're not a homogeneous compound the way most plastics are. The compounds going into a thermal cracking system, such as decane, are actually higher in total energy content than the products, such as heptane. They're just more difficult to work with, and as such less valuable economically.
Really, if you're not satisfied with the previous explanation you're going to have to look into an entry level organic chemistry textbook to understand it.
Tsuji et al. (2003) Thermal cracking of oils from waste plastics https://sci-hub.tw/10.1007/s10163-003-0090-x
Walendziewski (2005) Continuous flow cracking of waste https://sci-hub.tw/10.1016/j.fuproc.2004.12.004
Das and Pandey (2007) Pyrolysis and catalytic cracking of municipal plastic waste for recovery of gasoline range hydrocarbons https://core.ac.uk/download/pdf/53188820.pdf
What then? Just landfill it? Recycling mixed plastics is nontrivial. And old-school incineration is a nightmare of toxics emissions.
egg -> omelette
omelette -> egg
Some plastics can be recycled by simple melting - thermoplastics like ABS. Unfortunately they're often mixed with additives like flame retardants that cause problems.