Because it's much more convenient to have one single name for it in all of Earth's languages.
Here are the titles of the wikipedia article "Cerebellum" in some other languages:
Lillehjerne (Danish)
Kleinhirn (German)
Parengephaliδa (Greek - Παρεγκεφαλίδα if you can read Greek)
Cerebelo (Spanish)
Cervelet (French)
Otak kecil (Indonesian)
Smadzenites (Latvian)
Kisagy (Hungarian)
Beyincik (Turkish)
Xiaonao (Chinese - 小脑)
Nobody's copying the English word (well, Tagalog and Malaysian are) -- they're all using their own native terms for "small brain".They also use the word amongst themselves. Googling for 'cerebellum ugeskrift for læger' gives plenty of hits. Likewise for 'cerebellar ugeskrift for læger'.
They might write 'agenese' instead of 'agenesis', though.