Why flatworms don't have digestive track?

They can be microscopic, and they can lack a chest cavity or circulatory system. Because they don't have a body cavity, flatworms are considered acoelomates. They have simple digestive systems, with mouths to take in food and long digestive tracts to diffuse it around the body.

Parasitic flatworms do not have a digestive system because they don't digest their food. They just suck it when the upper gut muscles contract, suck it inside their body then gobble them up and then they're done sucking the nutrients from the food people eat.