> female vocal capabilities, which are objectively higher than men's

Not actually all that much among pop singers though. Most successful male artists tend to have quite high natural singing voices and females fairly low. And at any rate most music can easily be transposed as necessary. But I do agree there is a tendency by many male singer/ songwriters to stick to less vocally demanding melody lines. I'm less convinced that would be much of a deterrent for dedicated female songwriters hoping to pen tunes that were well suited to male voices.