Tons of common plastics are made from plants. PLA, the most common FDM 3d printing plastic, is made from corn. Ethylene in particular is also naturally occurring- it's one of the most important plant hormones.

Plastic implies it is solid, which is wrong.

many grades of peg are liquid at room temperature; the others are highly water-soluble. you've probably never seen an object manufactured from peg in your life. that's why people don't normally call it 'plastic'