I'm a marketer. I hold a Master's and a couple of Graduate Diplomas. I have a professional designation that requires constant CPD. I spend a significant amount of my time undertaking professional development. I work longer hours and can make a good case that my work has more impact than engineers, and I get paid significantly less than engineers with similar tenure.

I can't see any reasonable argument that engineers are intrinsically worth more, (and I say that as a person that dropped out of Engineering to pursue other opportunities). There might be fewer (good ones) than what the market needs, and this obviously pushes salaries up.