Because like you said, it destroys the body's ability to produce antibodies, in other words it destroys the bodies immune system. After this, the body is susceptible and helpless against any bacteria or virus it comes into contact with, and the body comes into contact with many, many of these on a daily basis, which would normally just be dealt with by the immune system, like bacteria which cause pneumonia
Simply because it kills your defenses, and without defenses your your body may not survive any diseases.
Also... Because it is a virus and not bacteria, as the previous answer slightly mentioned, antibiotics will not help -- for example: "the flu" constantly has newer, mutated versions because it, too, is a virus. To understand a little bit better... Check out this article.
Because with out an immune system you cannot fight them out
Because you have to much sex