The AOs are busy and might not have time to conduct the interview, or the recommendation letters and application are detailed enough to present a full picture.
International students (from most countries) have a potential language barrier, as well as cultural hurdles, so Harvard really likes to have interviews for international applicants to assess fluency in English, maturity, resilience, flexibility and personal qualities in general. International dmissions are rare without an interview.
A small percentage of admitted students from the U.S. have not had an interview. AOs only rarely do interviews of U.S. students. They rely primarily on the alumni network, and some areas have no interviewers within a reasonable distance.
Some areas, such as NY, DC, Boston, and SF have an excess of interviewers, and I don’t know why they aren’t asked to do Skype interviews of students from remote areas.