/bene/ is an adverb (near a verb)
/buono/ is an adjective.
In this case /to be/ is usded to join the subject to the adjective, that is why yiu need an adjective.
This special use of the verb 'to be' is quite frequent!
Note with another verb:
Parlo bene l'italiano (adverb)
Parlo un buon italiano (adjective)
Capisco bene l'italiano (adverb)
Capisco soltanto il buon italiano (adjective)
Capisco l'italiano, ma se scandito bene (adverb)
/essere/ has its special rules!