Because you dont get a second chance to make a good first impression.
-Do you want an adcom to open your application and, after looking at professionally dressed and professionally photographed applicants all day, sees you in a tee shirt in a bad selfie, what kind of impression does that make?
-Do you realize that at each individual school at least 80% of applications must be rejected prior to interview and the first impression you make apparently not even making the effort to be presentable?
-Do you doctors coming up to patients in tee shirts and jeans or in a tie/lab coat?
-Do you see that medical schools are trying to decide who will make a good physician and a picture is worth a thousand words, though a bad picture in a tee shirt only needs a few, such as "WTF?" and "poor presentation" ?
-Do you have any idea what I am taking about as, this should be inherently obvious to anyone applying to a professional position as it shows, on its face, an understanding that you behave, act and appear professional?
-Do you know that I find it truly amazing and utterly baffling that bright, overachieving, allegedly mature college students who are applying to medical school need to be told any of this?