First, many people are pretty comfortable with not talking very much, for various reasons other than having low self-confidence or bad social skills. Or at least they are comfortable until someone makes a point of how little they are talking and tries to awkwardly force them into spitting out more words for the sake of words.
Even the people who do just lack confidence and wish they could be more chatty don't usually appreciate the kind of "help" usually offered, which might involve suddenly putting them on the spot in a group conversation, or pretending one of the few things you know about them is suddenly a very interesting subject to you.
"So I hear you work in an accounting office! Tell me all about it! I bet you have a ... desk ... and everything!"
Other well-meaning tactics involve pushing them into some embarrassing party participation role, like forcing them onstage for karaoke, making them chug something, pushing a stripper on them, or whispering to the waiter at T.G.I. Friday's on their birthday so the whole staff will come over and sing one of those humiliating songs, all in the name of getting the person to "loosen up" and "come out of their shell."
Even worse, when they do gamely participate, the horribly awkward results are often applauded with extremely fake and over-the-top enthusiasm, as if the person's budding confidence is extremely fragile and every sign of progress must be heavily nurtured.
"Man, you really ROCKED that version of 'Don't Stop Believing'! Who knew you were such a wild man!"