I’ve mentioned on other threads but will repeat here. First I’ve done many interviews over the years for a top school. With that said unless someone here is an AO and can comment on the role of the interview everything here is speculative.
Over the years I have seen many excellent candidates, with excellent credentials, and equally excellent professionalism with respect to the interview (prompt response, great interview, great knowledge about the school or program they are applying for, great conversation , engaging questions, a thank you note etc) resulting in a stellar interview report…get rejected.
Over the years I have also seen some very poor (in my eyes) students who have been unprepared for the interview, knew little about the school or program, did not seem to stand out in my eyes, may have seemed disinterested, may have been tardy, may have not written a thank you, and have been evaluated unfavorably by me…get accepted.
I have also seen some candidates for whom my report said "could not reach candidate despite multiple attempts '(usually up to five using email or phone) or “declined interview”…get accepted.
The mystery of the process remains and the role of the interview is unclear.
So when schools say the interview is optional they mean it. As an interviewer its a bit frustrating to feel at times your input has little impact especially if a candidate has such a negative impact on you ( there are many great students so the opposite is not as big a deal).It may be why OP mentions its hard to get enough interviewers and retain them to reach candidates as some interviewers get frustrated at the results of their interviews.
Some have even speculated the interview serves more to stroke the interviewers ego and keep them interested in the school ( and possible donations) than it does to serve the candidate who can readily find out information about the school or program elsewhere. As most interviewers are not necessarily recent grads or directly connected to the current school a candidate would learn more from a recent grad or a current student or faculty member than an alumni interviewer who went to a school years ago, a school that is very different from the version the applicant is applying to.
With all that being said other than an AO no one really knows the role of an interview. That aside if you do an interview simple courtesy is nice.
Bottom line is students shouldn’t worry about the interview and be themselves. Often times the decision to admit has already been made (school wants a particular candidate to fill a program, quota (minority, first gen, national or international distribution, financial considerations, athlete etc.). Have fun, be courteous, and professional you never know how far it will get you.