Plastic is a polymer which means it has same structure repeated throughout the material at atomic level. Plus, It is flexible regarding shapes and size. It is same when tissues are developed before surgery. Such tissues are chain of same kind of cells duplicated from a sample taken from same person's body part on whom such surgery has to be performed. Such tissues are also flexible just like Plastic.
Hence it is known as Plastic surgery. :)
Nice question. I have little bit of knowledge about biology which I am sharing.
thanks for the opportunity.