Hey, maybe people just don't see the benefit. So either it doesn't exist, or the unions have done a thoroughly lousy job of being convincing.
Why should these shops unionize when the basic issue -- wages and benefits -- is moot? Wages and benefits at the "new big three" are equivalent to those that the UAW negotiates for the "old big three" ...
Also don't forget, the auto industry is unionized in Japan.
/jordan