How come Mother Nature never invented plastics all on her own? Seems to me that there must have been some kind of chemical reactions over the course of billions of years that might have caused plactics to be formed. Any chance that one day we’ll find a big vein of HDPE somewhere deep in the earth’s core?
Mother nature did indeed invent plastic. Place pine resin in seawater for an extended period of time and it will polymerize into extremely long chains. This material is known as amber.
Why is there not other kinds of plastic that are naturally-occurring (or are there)? Also, if it is possible to make plastic, is it also possible to deconstruct it before it goes to landfills so that it is disposed of in forms that more quickly degrade?
Unless I’ve got my facts mixed up, aren’t our brains essentially polymer gloop?
One of the reasons is that industrial plastics tend to be made up of one or two monomers. Impurities, which would likely occur in nature, can cause a polymerization reaction to come to a halt. Or the resulting product wouldn’t be a nice clean polymer such as we are used to. Also, many polymers are made under extreme conditions (high temperature and pressure) and in the presence of catalysts, such as metals like nickel. Having all of the conditions in nature would be extremely unlikely.
Another natural polymer is latex rubber. In fact, the synthesis of artificial rubber is widely regarded as one of the biggest chemical achievements of the 20th century. The impetus was Japan’s control of the south Pacific during WWII, and the need for rubber for all sorts of war-related products, including gaskets and seals for engines, waterproof gear, etc.
Gadfly our brains are essentially lots of cells. Now, biopolymers found in our bodies, such as nucleic acids (RNA and DNA), proteins, carbohydrates, and fats are not thought of as plastics. Biopolymers and plastics are both types of polymers, but they are not synonymous with each other.
Natural polymers include protein and cellulose.
Those are pretty big groups, aren’t they? 
Besides, I don’t think there would be an evolutionary advantage of making something that would last for thousands of generations (as in the case of man-made platics) and affectly leave the carbon cycle.