It’s not marginal utility it’s supply vs demand. The reason SWEs can demand so much is because there are a lot of options to branch out solo or small company so companies need to pay competitive wages. The employees are therefore able to capture more of their value.
For chemical or mechanical engineers they need a lot of infrastructure to do work so employees are more locked to employers and wages are not as competitive. Employees are unable to capture their value without the employer therefore they get a smaller piece.
Also SV has an obsession with hiring the best while most large manufacturers consider heads interchangeable with only a few spots reserved for top talent.