Why is opera always sung in Italian?

All opera is not sung in Italian, but Italy is much an opera-associated country and plays an important part (like Teatro alla Scala) so that’s why we make such connections.

Italian is a particularly good language to sing. Italian has a lot of vowel sounds compared to French which has a lot of consonants. You can hold a note on most vowels but not most consonants. While Italian is a good choice to write pretty songs there are French language operas too. As others have said, operas are performed in the language they are written in. Considering that most libretti (stories told in operas) are silly it isn’t that important to know exactly what the performers are singing; if you know the gist of the story that is usually good enough. Wagnarian operas (my favorite) are German. Wagner also based his operas on Norse and Germanic mythology rather than the silly little stories in most operas. Purcell’s Dido and Aenaes is considered the only great English language opera.