Let's give them a break. I suppose the most exposed context for modern composers is movie sound tracks. If that's where the money is, then it stands to reason the music would all sound like that. Opera is not a movie. The music is only partially there to provide a background for the action. In opera the music is the action, in particular the singing is the action.
Singing hasn't changed since the early days of opera. Except maybe tenors have pushed up their chests. Make what they're singing sound interesting. This is the last thing a modern composer thinks about, and sometimes it's the next thing after the last thing.
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