The point is that if you go to a target school doing Math olympiads throughout your school life, the admissions exam and interview is a walk in the park. The applicants who didn't have any of this preparation can still do well but will fare relatively worse against that group, and I think this is very obvious in the ultimate intake.
The interview process is relatively standardized, and if my results were to differ starkly from what more experienced interviewers do they would be disregarded, and the director of studies for that college would simply not invite me back to help.
I would add that asking PhD students to do this is not the worst thing because via supervisions and other teaching efforts, we have a good picture of what undergrads here need to be able to do. An interview is a like a short supervision.
I don't believe anyone wants the course to be easier. So how do you enable people who are underprivileged to catch up with a shitload of additional tuition which has been offered to other applicants? How do you distinguish effectively between the two, whilst also accepting high aptitude students?
It's not that I think any elite university does this brilliantly - but as someone with a decent level of exposure do you have a feeling on whether there are complete solutions?