I believe this was asked in UChicago and Pitt threads. Both of these schools are notoriously fast at selecting students, positively or negatively, for interviews. While they do do it quickly, it was assured that applications are looked at thoroughly and selectively, but that each application does get an AdCom's eyes on it and filtered, even though some applicant's got interviews or rejections within a day of submission. To view an entire application that these students put years into making, especially if you're a practiced reviewer, can only take minutes. I think it dissuades people from thinking they got a thorough review if it goes this quickly, but I appreciate it much more than the schools that are still silent for me. They obviously have reviewed, and decided against, my application. I think it's much fairer to let the students know. I see this justified in applicants that are 'outliers' in stats. If you look at the UChicago or Pitt threads you will see applicants getting swift rejections with 3.8+ 35+, and others with 3.6-3.8 and 30-34 getting quick interviews. How would they ever select these applicants this way unless they review the applications thoroughly (and yes, these are non-URM applicants)?
And of course, the secondary fees do help fund the school. But they lose money on interviewing students. If they interviewed you they were probably looking to see if you were a spectacular "something else" they were looking for that could have overcome the lower MCAT. When they didn't see that, they went for the 'easier' out to reject you because that didn't come out. They say it was your MCAT, because if you had a higher MCAT maybe you wouldn't have needed that "Wow!" factor at the interview. I don't know for sure, that's just speculation. I wish you the best this cycle though, sorry 🙁