The stereotype here in America is that the extroverts are the outgoing, outspoken, natural born leaders. Whereas the introverts are often stereotyped as the shy, quiet, yet intelligent, more artistic people. These stereotypes are not really correct, but they are treated as such. I think it is portrayed that introverts have been some of our favorite people in history, but regardless extroverts are treated a little better generally.
Yes, Americans value extroverts much more than introverts. Extroverts are seen as more fun, more likable and tend to have more opportunities. When you apply for a job, having a fun, bubbly, charming personality is pretty much a requirement. People who come across as shy or quiet won't get the job over the outspoken person. It is unfair to not give someone a job simply because they are not as bubbly or as loud as the other person. Introverts may be more reserved
but that doesn't mean they shouldn't get a job they are qualified for.
American society is all about valuing extroverts over introverts. In the United States, someone who just wants to keep to themselves is seen as being defective. We seem to have some national sickness that makes us value people who draw attention to themselves over those who maintain quiet dignity. We really should work on changing that.
Americans value extroversion more. This is because the country is super-capitalistic and capitalism often highly depends on aggressive, outgoing people to make money. Introverts do also have a place in the United States, but they are not quite as valued as the go-getters that fill the corporate and political arenas.
I believe that Americans value extroverts more than introverts, because people will big personalities are more likely to get ahead. This is true even if the introvert is better at doing the job. It is true even if the job is something that requires no personality. Extroverts are preferred because they demand more attention.