Why can't we use the chemical compunds of our plastics to power things such as cars?

We can!!! It’s just a matter of cost, availability and motivation. Efficiency is always a consideration for the production of anything, including fuels. Having a market is always a consideration for developing a process for anything. It is virtually impossible to beat coal, natural gas, and petroleum feed stocks.

Reference:https://www.quora.com/Why-cant-we-use-the-chemical-compunds-of-our-plastics-to-power-things-such-as-cars