I agree. Most Engineering admits at many of the best schools have dramatically better stats than those admitted for humanities. And yes, typically, it is not even close. Engineering is a significantly higher paying field and is therefore more competitive plus it attracts more Asians who have higher scores & GPAs (and before I get accused of reverse racism - there is not even a doubt about this. please read the research out there) so it gets further skewed.
In engineering admissions, only grades, scores, a mathematical aptitude and a desire to be an engineer should count and I believe regular essays and ECs are not great indicators of success. Do you really care if a NASA rocket scientist has learnt life lessons through Art for example? His job is get the astronauts safely up and back and the closer he is to a computer, the better he is for all of us. Why bother with an essay? Will it help him design a better manifold? There should be a separate admission process for engineering majors in particular. The standard applications are pointless. My 1st degree was in Engineering and i find the common app or the UC app format pointless for engineering applicants.
Holistic admissions, meaning essays and ECs are actually the only way to cull from a sea of similarly scoring kids when it comes to Humanities though. SAT scores are actually not great predictors for Humanities etc but beautiful essays absolutely illustrate the intellectual prowess for such students. For students applying to non-STEM non-Business majors, there is no point in using their SAT Math scores for example to determine how good they will be.
Unfortunately such distinctions are hard to make by adcoms poring over thousands of applicants so we have a hodge podge of an admissions process where engineering applicants need to have volunteer hours and SAT Math scores are considered relevant for sociology majors, both of which are wrong. Add to that is the fact that changing majors is easy and is expected to happen for up to 70% of undergraduates and you end up with a holistic approach for all.