I think in healthcare in general we look down on people that can't "suck it up" and come in. I know I am personally guilty (I have used 4 sick days in 18 years. Honestly, three of the days I was sick as hell, and the fourth I was in mgmt- and just annoyed with life and took a mental health day and went and drank beers and hit golf balls with my buddy. That day kind of is microcosm of why we don't use our sick days that much. We don't want to (or conversely don't want others) to call out sick and put more work on us. That day nobody had to cover for me, the work just stayed there until the next day for me. For many of our non-healthcare friends, that is common place (obvious exceptions such as retail, etc).
Secondly, with all of our time off (at least for me) in one bank (vacation, holiday, sick) - if I am able to work, I am gonna come in because I would rather save my time off for when I feel good and doing something fun (I know for most doc's this isn't necessarily the case as many are not direct employees of a the hospital and you don't necessarily have dedicated PDO)
edit- for one year when I was right out of school I was an employee of a super small hospital where I was the only pharmacist. I had sick days that were use them or lose them at the end of the year. Knowing I wasn't going to be able to just randomly call out, I had a few prn semi-retired rphs, I conveniently was sick every other friday for the last two months of the year and had an agreement for one of them to cover me. I worked for a staffing agency, and felt zero remorse taking what I was due based on my employment contract.