FReeper Paul Ross has a nice graphic that shows the amount of secondary jobs spawned and supported by manufacturing activity, compared with service industries. The manufacturing support jobs are, IIRC, something 7 secondary jobs to each manufacturing job. Service industries are lucky to have a 1:1 ratio. There simply aren't enough things you need to do to run, say, a dentist's office, compared to the things you need to do to keep an electronics plant going. Not to mention the difference in wages.
We have to have something for the service industries to service, is the bottom line.