Why is it better to use plant and animal based products in place of petroluem and plastics?

I know that for animal products, maneur can be used as fuel in third world countries and that they are better for fertilizing. I know that animal and plant products are renewable. I know trhat corn plastics are biodegradable.Ultimately I know animal and plant products are better because seeds can always be planted , resources can be used over, and because more animals can be created.What is the difference in manufacturing for plant and animal products versus petroleum and plastics? Do you have any other reasons I can add to why plant and animal based products are better especially beacuse of environmental impact?