Because biology is a topic of its own. Think of it as "I understand the biology of human" vs "I understand the biology of humans". One makes sense, the other s

Because biology is a topic of its own. Think of it as "I understand the biology of human" vs "I understand the biology of humans". One makes sense, the other seems meaningless, not grammatically correct (and so blatantly it sounds terrible to a native speaker) or at least makes you ask which human.