Basically because that's what your contract agreed to. I have something similar. I have 3 sick days per year and if I don't use them they become vacation PTO for next year. Regular pto doesn't roll over but sick days do.
There are advantages and disadvantages to both approaches. With combined sick + vacation, usually called PTO, workers are more likely to try to save PTO for vacation and work when they are sick, and they end up transmitting infections to other workers. Personally, I am a big fan of separate vacation + sick leave.
The counterargument is that some people abuse sick time. (I think there have even been studies that show a large excess of sick days on Monday.) And from a purely utilitarian perspective, why should an employer care if you're in bed with the flu or climbing a mountain somewhere?
That said, I'm not a fan of the combined system. In a lot of other ways, we expect employers to accommodate employees who may have issues of various kinds. Accommodating employees who get sick more often seems a reasonable extension of that. (And I say that as someone who, touch wood, has largely benefited from a combined system over the past decade or so.)
> And from a purely utilitarian perspective, why should an employer care if you're in bed with the flu
Because the alternative to being in bed with the flu is being at work with the flu, and the employer doesn't want to catch the flu? It kinda makes utilitarian sense.
What I meant was that you're not working so you should simply forgo that day of climbing the mountain. It's a day off work in any case.
As I said, I'm not a fan of the combined vacation and sick time in any case (in part for the reason you say). And, yes, people will drag themselves into work because they don't want to lose a vacation day.