In short it spurs from not being a winner.
The socially anxious person maybe knows at the intellectual level that social wins can be achieved by working the numbers game, but they are too apathetic or nihlistic or scared that their first attempt is going to be a loss which will further depress them.
When you immerse the socially anxious person in a controlled environment you make sure that his first attempt is a Win, and the second and the third and the fourth etc.
And you make them win enough times that they maybe venture into hubris, so that they have a full tank to go into the real world and face the natural X amount of rejections and losses which comes before a win.
I disagree about the win/loss theory in general though. I think it is really a dissonance about what people expect and value in in or out groups.
Someone afraid to speak up in certain groups might be quite outgoing in another. So there isn't really an absolute level of success. There are also very successful people that are anxious. Expectations towards yourself play a large role though.
Everybody curates their environment, people just do so unconsciously and without realizing it. An American movie star might dominate the room in Los Angeles...but they'd find a hard time even speaking to people in Iran, and if they are recognized people would be hostile towards them.