Because we guess that the sentence reffer to something in the previous one. Else, we could also say "Il l'a beaucoup aimé"
We could have those sentences equally correct:
Il a bu un café. Il a beaucoup aimé.
Il a bu un café, Il l'a beaucoup aimé.
But maybe we can also have "Il a beaucoup aimé" to resume one, two or many sentences.
Example:
Il est allé à Paris le weekend dernier. Il a visité la Tour Eiffel et le Louvre. Il a visité l'exposition de Tim Burton et ensuite a bu un café dans un endroit chic. Il a beaucoup aimé!
(= I liked a lot all of these things)