Opera musicians know much more about the physics of their trade than amateurs like myself do. But they are on a different wavelength than Broadway musical singers, who themselves are different in their genre than popular music stars. The Carpenters did not have to conform to demands of other arrangers when the brother could shift the keys and the tempo to find the best one for any song such that the vowel sounds and breathing fitted the resonances of his sister's voice.
Perhaps you have not experienced the sense of muffling and stifling that comes from having a melody played in the wrong key (even a half note the wrong way) and/or in an environment for which the resonances are not fit for the song or one's voice. Matching the elements is a science of its own, for which the composer can only partially compensate. You'd be surprised why so many songs are arranged in B-flat, F, or G for the Western human voice.
There is much more about the business of musicianship than the lay person is likely to comprehend. Living with it as a source of income and talent development sets the practitioners apart, often not able or willing to go into all the details with the consumers of their art.
Where do you think the Great American Songbook came from? Mostly Broadway.