The 'meat' of being a professor keeps me busy enough that I am simply not interested in getting involved in the business of excuses: one student misses the exam because of a hangover, and another because of an attack of multiple sclerosis. Clearly there is a line (fuzzy or sharp) distinguishing legitimate from not, but I just don't care where it lies.
I'm not interested in wasting my mental resources deciding where to place the line, deciding whether any offered excuse is offered under false pretenses or not, and indeed, discussing any excuses. Now, 'Hang together' prof may retort that she is able to do both: refuse to drop exams, and not waste any time with offered excuses, but I'm just a different person (is it my Catholic upbringing?).
If I refuse to drop exams (as I did during my first semester teaching), when students offer excuses I'm wracked with guilt, and the time spent wallowing in guilt is time wasted. Indeed, one recent student missed the first exam of four because of an epileptic attack that occurred during the time that the exam was scheduled, took the second exam, but then during the third exam had a panic attack (a nurse at student health called me just after the exam to tell me so).
These may sound like textbook examples of malingering, but I'm actually perfectly willing to accept that they occurred. Even though I felt justified refusing to allow a makeup for the third exam, I still wasted plenty of time wallowing in my guilt. I can't say I'm proud of it, but that's just the way I am. So ultimately dropping exams reduces the number of requests for makeups, gives me leverage to refuse to drop any further exams, and though it doesn't eliminate the resulting knot in my gut, by all means it does reduce its severity and duration.
Not only that, but it allows me to give an extra little bonus to the best students: they've typically done well enough throughout the semester that they can drop the final exam. From my perspective, there are really a bunch of benefits to dropping the lowest exam, and not one of them has to do with student evaluations.