You may find that some of your friends took up an instrument precisely because they were introverted and needed something to fill up the time they were spending on their own. Or they wanted more friends but had decided for themselves - or, in some cases, been told directly to their face - that they were 'boring' and they thought being in a band would help make people like them. In later life, they might even admit to being too shy to perform in public, much like some people have a fear of public speaking.
Some just play an instrument for the sheer joy of it - their equivalent of a computer game or other absorbing hobby. They could be very secure in themselves and have no desire for fame or fortune.
Personally, I think it's a shame that so many able musicians aren't getting themselves seen and heard because there are so many small towns where people are 60+ miles away from the venue their favourite band is playing at and the petrol from one trip means they can't afford the ticket price on another band.
Gentle coaxing of your friend to the nearest available open mic night would help but don't attempt this unless you're prepared to turn up on the night yourself and bring other friends with you.