Like Kate said, antibiotics don't always work well. Some doctors would argue that they don't work. (which obviously is disproven is this thread ahah)
Generally, they give them anyways, because of the possible system infections. But it is relatively rare for the antibiotics to heal them on their own... I think it might have to do with the state of the tissue, how much the abscess fills, and the body's response... Some people can pop an abscess, take some antibiotics, and it doesn't fistulize or fill up again. But, this is unfortunately rare.