The singing voice is based upon a reflex action which is built in to the body. Speech on the other hand is a skill learned when we are young, which re-purposes the mechanism of the voice.
Our job is twofold: Firstly to learn to guide the voice so that it produces the sounds which fulfil our artisitic vision; secondly, to augment the natural singing voice with some additional sonic palette derived from speech sounds.
In learning to sing we are learning firstly to free the voice from the shackles of speech habits, bypassing speech to allow the body's natural system to take control of many aspects of singing voice production without the intervention of linguistic thought or action.
In passing control of primary voice production from the conscious mind and the speech-centre over to the singing system, we are allowing the body to focus the resonance of the sound and implement the pitch in much the same way as the eyes are automatically focused.
Just as we would struggle to see properly if we were to attempt to assume control over the focus of the eyes, we struggle to produce the 'cry' (singing voice) if we attempt to control the vocal tract either consciously, or we allow the speech habits to control it.