Why do female opera singers mostly sing in the head voice? Is this a stylistic thing, or is it because it's harmful for the voice? Is it wrong when a mezzo-soprano sing mostly in the mixed or chest voice in opera?
The short and simple answer is: because this results in a vocal sound that opera fans like to hear. Singers develop certain techniques because the vocal sound that results from these techniques are popular.
Furthermore, composers write music that requires singers to be able to use certain techniques.
And also the operatic style requires a singer to be able to be heard clearly without using any microphones or amplification, while singing along with a large orchestra, in an auditorium full of more than one thousand listeners. So opera singers develop certain techniques to enable their voice to be heard clearly under these challenging circumstances.
The techniques that one individual singer learns are those that are appropriate according to the singer's gender and the type of voice that the singer has, whether bass, baritone, tenor, contralto, mezzo-soprano, or soprano.
They don't mainly use head voice, actually. If it seems like this is the case, then it's just a factor of where a piece's tessitura lies.
In classical singing singers tend to sing much higher than in other styles and genres. Thus while they may be using head voice proportionally more than non-classical singers, when they sing in the same lower range they always employ chest voice or a mix.