They're actually not in Italian, most famous operas predate the italian language you'd be taught in schools.
People who can speak the dialects those operas were written in are apparently now more common in the Americas than they are in Italy, as Italians who encounter self proclaimed native italian speakers from the Americas consistently complain that either they're speaking some incomprehensible other language or that they sound like someone's nona with how weirdly old fashioned they're talking.
This is actually a pretty funny phenomenon across multiple languages that have undergone standardization in the nationalist era, even including speakers of Chinese languages and Japanese, the rooted well established communities often predate standardization, and so the language "native speakers" learn in modern times can actually be incomprehensible to modern people in the place the language came from.