A lot of it has to do with lyric writers who dont know how to write for the human voice.
Opera librettists are usually much more in tune to that thing and you'll see better text setting for those reasons.
Vowel modification is just something singers have to do in order to get the sound out and still remain healthy, remember these singers are usually doing 8 shows a week, or at least 6, so anything to make it easier.
Conversely, opera singers don't often have to perform the same role week after week. An opera singer who has his/her druthers and performs often will often peform a heavy role (like Verdi or Wanger) and then lighter roles (Mozart) in between, it keeps your technique fresh and keeps your voice healthy.
Musical theater singers dont often get that luxury, so they need all the help they can get.
Cut em some slack guys.
"Fenchurch is correct, as usual."
-Keen on Kean
"Fenchurch is correct, as usual."
- muscle23ftl