As oil it was trapped under shale or in sands where it had no effect on the water table and didn't get into the water or food supply.

As plastics it is "broken down" into microplastics, and gets into things and areas it was never intended to be. It is then consumed and builds up in the systems of animals, and even eventually by us.

Even your local landfill, is part of a management system, but will it really last 3,000 years?

We can't just keep adding and adding to it, we can't cover the planet in landfills of plastic, we aren't putting the "oils" back to where they came from.

In Canada, I remember being taught "give a hoot, don't pollute", and to us, as kids, that meant "don't create trash" as much as "clean up your trash".