After 8 years of teaching in HK. at language centres and in a private capacity I suspect that the reason interviews are conducted are to eliminate students that could negatively affect the school's image and also to reduce potential parental complaints. If schools accept a special needs student for example then local HK. parents have been known to complain that the student will cause their kid(s) to have their education dragged down because now the SEN. student is receiving extra help at the expense of their kids or the parents will complain that they don't want that kind of student in the same class(es) as their kids(s) which leads to said SEN./"disruptive" student(s) being kicked out of school/kept at school but denied extra assistance. HK. parents have been known to threaten to withdraw their kid(s) from schools with other "less desirable" students and will use that as leverage against learning centre owners.
Having been a student at a private British college, these kinds of institutes only see pound signs/$$$$s, if there is even a whiff of scandal or dissatisfaction from fee-paying parents then every effort will be made to appease them and keep their money.
The interviews prep. system just makes people running these courses very, very rich. I've prepped. and designed courses and materials to teach these young children what amounts to pre-interview etiquette courses before, if their parents will spare the time to teach them manners at home then maybe these companies wouldn't be so prominent.