Honestly big student loans are the problem. Kid one goes to college, graduates with a degree in musical theater, has a $120k debt and gets a $50k first job in marketing. Kid two does two years an a plumbing apprentice. She's been making $45k per year, has no debt and just got her license... And is now making $90k.
And don't kid yourself, any given city where a marketer makes $50k, even before commissions, will not be seeing plumbers make $90k.