I know it is not good form, but see my post #67. It also makes the point that admissions has specifically avoided testing whether essays either correlate with later performance, are reproducible (different adcoms give same scores), or change the admissions decisions (by doing a subsample both with and without).
The reason is the whole drama helps elevate the process, and gives everyone an illusion of more control. Individuals submit their own essays, not just a test score, universities have whole admissions committees considering each application, and it all seems so much more fulfilling than just grades and test scores. A school that ditched essays would be seen as unserious and impersonal.