
Because even if you were adjusting the moonphase for months less than 31 days, you'd only be doing this five times a year. And even this is not necessary as one could (and should) simply depress the button on the side of the case with a wooden toothpick to advance the date on months shorter than 31 days without affecting the moonphase.
Perhaps you are having to adjust the moonphase every month because of something else?