Dear student,
Simple organisms such as Hydra and Planaria are capable of producing new individuals through the process of regeneration. The process of regeneration involves the formation of new organisms from its body parts. Simple organisms can utilize this method of reproduction as their entire body is made of similar kind of cells in which any part of their body can be formed by growth and development. It occurs by mitosis.
However, complex organisms have organ-system level of organisation. As regeneration happens through mitosis and only a particular type of tissue can give rise to its own kind. Therefore complex organisms, that have different tissues and organs have altogether different structures & regenerating a different kind of tissue from another kind is not possible & hence reproduction by regeneration is not possible.
Regards