It's yield protection. It's the same reason why a lot of schools dont tell us our position on the waitlist even if it's ranked. They want to have their cake and eat it. If a school told people on the bottom of the waitlist and they had no chance, people would withdraw and even though there's a low % chance of actually admitting people on the bottom of the waitlist if that happened and they couldn't fill the class they'd probably lose a bundle of money.
In the case of AZ, they're prepared in case a lot of people withdraw even though they interview an absurd number of applications and it's not fair to us. The emails also feed into our false hope and can easily be interpreted to mean "wow, my application is getting reviewed again and again, I might have a chance", even though that's not the case, so we stay in the pool of applicants available should they need more applicants to admit. They're also medical school admissions and medical school admissions can do almost anything they want and have their cake and eat it, and we're stuck dealing with the ramifications because there are so many people applying to med school who will jump through these hoops that our thoughts are totally inconsequential and there are thousands of people to replace us. Logic about what happens and how applicants feel goes out the window.
In fact this entire medical school admissions process is a testament to the fact that medical school admissions can and will do whatever they want most of the time with no thoughts about applicants. That's why medical school admissions aren't transparent, that's why they can lead you on for months, and have you fly across the country, have you sit through 9 hr interview days to be potentially rejected and given no information. If you think being led on for months is unfair, what do you think about your entire future boiling down to often one 30 minute interview and what one person may think of you, regardless how potentially biased that person may be and interviewer inconsistency? What do you think of how a difference of a few points on the mcat can mean the world to medical school admissions or even a small gpa difference can too? (At one school I was rejected because my gpa was .o2 below a cutoff that isn't posted anywhere) None of those things are fair but it's the process.
Medical schools can reject you for any reason, treat applicants however they want, and because so many people want to be doctors so bad, we are at the mercy of the system. It isn't fair and it's a harsh, but those of us including myself who are willing to and want to be doctors bad enough will keep going through the process no matter what. I really do wish medical school admissions would be more transparent, be more open to feedback, change, and being more user-friendly because they put a large amount of unnecessary stress on us, but we have to go with it because we have no control over this and we want to be doctors. You can find lots of double-standards and holes in admissions logic, but that doesn't matter because their rule is the law.