What about those who want careers involving higher education? I graduated from college and graduate school early (AP tests and joint degrees, respectively) and I STILL didn't enter the workforce in any real capacity until the age of 25. All my summer and part-time earnings went to basic living expenses. Sure, I COULD have done some sort of semi-skilled task prior to that time, but at what cost? My grades would have suffered, and I may never have gotten my current job (which pays many multiples of the semi-skilled professions you mention). It might have increased my net worth at 30, but significantly decreased my net worth at 40.
And why the focus on 30? Isn't your net worth at 60 FAR more important?