They advanced so far that medicine and raising/growing food has become obsolete, with all other animals and plants going extinct along the way.
If everyone has been genetically modified in the past to have perfect health and exceptional recuperative abilities, and those changes have successfully propagated for long enough, then over time medicine would become increasingly obsolete and the knowledge about human biology would be lost.
If all food is synthesized because all animals and plants have gone extinct, then there that explains the lack of knowledge in biology outside of humans. Unless your pet needs surgery, but let's face it, most effort into biology outside of the human body is for food purposes.
Artificial intelligence
AI was developed that handles things everything and propagates the knowledge to other versions of itself, so humans neither need to apply the knowledge themselves or know it to propagate it which would result in its loss among humans. Not lost in general, but no one knows what the machines are doing to get things done.