I believe this is because reading by yourself is not a form of communication. *You* aren't trying to communicate message to anyone or anything, not even yourself. Your eyes are doing that for you. Speaking *as a form of communication* is something unique in that it places stress on the speech system due to communication being structured through rigid rules of vocalization or "language". Any deviation from these language rules is potential miscommunication, and the stress is on our speech system to follow these language rules correctly to convey our intended message. word switching is a way to circumvent the pressure of these complex rules, allowing us to convey the same message with more variability in the way our vocal cords can produce the sounds. Notice how then you're blocking and someone says the word for you, all of the sudden you can say it? There is no longer *communication stress* attached to the word, due to the person you're talking to already knowing the word you were going to say. Singing also allows for great verbal flexibility, unlike language, and allows sounds to be stretched indefinitely so that the vocal cords won't spasm under tightly timed constraints like normal, spoken language.