Why would someone think that test prep is readily bought, when the student who was prepped still has to take the test alone, while essay prep can’t really be bought, when there is no real barrier between the tutor and the final product?
The essay survives, and in fact really hasn’t been tested, because it is in almost all stakeholders interests for it to survive. We desperately want to believe the whole process has a personal element, and the essays help us hold on to that belief.