You've obviously never had to organise & administer (high-stakes) examination interviews. It'd be very expensive & time-consuming for already overstretched education institutions. It's just not feasible.
I have, without any teaching assistants, organized and administered high-stakes examination interviews. Each meeting used up about 30 minutes of my time in my office, which is not too different from how long it takes me to read and grade a written test or term paper. This was in a large-ish US university, where students get nervous about that sort of thing. I can confidently report that due to my careful use of follow-up questions, I can get a very clear sense of the depth of each student's knowledge.
Science may someday discover what faith has always known.