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.