Internal parasites originated from free-living organisms, but the process of reverse transformation

Internal parasites originated from free-living organisms, but the process of reverse transformation of internal parasites into free-living animals is impossible. Why?

Parasitic worms have lost some organ systems because they are unnecessary (the respiratory system, the digestive system in some worms) and acquired adaptations (the cuticle is a dense body cover that protects the worm from being digested by the host’s digestive enzymes; suction cups – to fix it in the host’s body). Due to the general degeneration (simplification) of the worm’s body, it cannot become free-living from the parasite.