This isn't EXACTLY true and I think a lot of people don't realize it. The thing about interviews is that SOMETIMES they don't have much to do with the overall admissions decision. They're just sort of a standard procedure in order for the admissions committee to fully consider your application. But in the end, it could still be your stats and extracurriculars that hold you back. Two years ago, I interviewed with zero shadowing, zero research, and minimal volunteering and STILL got two interviews but no acceptances. The reason I didn't get accepted had nothing to do with my interviews, I feel they went really well. And I'll admit I was a little annoyed that the schools interviewed me knowing full well the limitations of my application and that they really had no intention of accepting me. Even this cycle, one of the schools that interviewed me ended up not accepting me due to lack of volunteering, even though they knew damn well going into the interview I didn't have it. According to the director of admissions, I didn't really stand a chance because of it. This one was a bit more aggravating since I had to fly out to the school.
Anyway, yeah, I'm not an expert, but sometimes I think schools interview you even though they know you don't have much of a shot. I call it wasting time, I guess they see it differently. But just because you don't get accepted somewhere doesn't necessarily mean they don't like you as a person, they just don't like your application. I had an interview this past week. My interviewer really liked me and told me he'd do his best to help me get accepted because he's on the admissions board. But there is STILL a reasonable chance I WON'T get accepted due to something in my application that isn't good enough for them. It's hard. The application process is damn hard.