Fear of not matching. Even though the advice is apply to 30-40 and interview at 12-15 (assuming no red flags or couples match), I'd bet that most students don't want to take the chance and would rather be overcautious. There's too much at risk, and humans are frequently risk averse. Logically, there's no reason to apply to 100 programs and go on 20 interviews, but sometimes the irrational or emotional side cannot be convinced.
As discussed endlessly, SLOEs are probably the biggest factor. Although we've been given some insight on how the SLOE is generated (thanks to a fantastic aPD), not every program may operate the same way. From my own experience and talking with friends, I know some rotations only have you turn in 2-4 shift cards while others have you turn in 14. If you're only turning in 3 shift cards (blinded), that seems like there's very limited information to even write a SLOE from. Sure, residents and attendings may be discussing students privately outside of the shift cards, but as students, we don't get that feedback. We're also at the point where we may be too cynical. We've all heard about students receiving excellent feedback in person and on school evaluations only to end up with a sub-par or outright damning SLOE. We've heard about students who were told they would be ranked to match or at the top of the list only to not match at that program or even worse, fail to match completely. We've been advised to take everything anyone says (good or bad) with a grain of salt.
Furthermore, some programs may heavily weigh where the SLOE is from. Not all students were able to obtain rotations at well-known/established programs. If I got an excellent SLOE from a lesser known program that recently switched to ACGME, would it still be weighed as positively if I were to get one from someplace like Indiana or LAC-USC? Maybe, maybe not. At the same time, if I received a poor SLOE from that same lesser known program, my application is down the drain. What if my SLOE writer doesn't have much experience with them? Might I possibly end up with a poorer SLOE? We've been told repeatedly that SLOEs are the number 1 factor in the EM application. Unfortunately for students, it is the one thing we are (mostly) blinded to in terms of how it is generated, what evaluation we received, and how much the reputation of the program that writes the SLOE really matters.
The bottom line is that many applicants are deciding to be risk averse due to fear and uncertainty. Students have spent 4 years and hundreds of thousands of dollars getting to this point. We'd rather drop a couple grand more to make sure that we match in our specialty of choice especially if you're a DO, have average scores and have an uneasy feeling about the SLOE.