Because the need to use crack/heroin/meth overrides anything else.

Most of the time it happens when they are young and they believe that its okay. Take it from me... I know.. We feel useless and like no matter what we do we aren't good enough. I never had anyone to talk me through it but my school made me talk to the counselor. I won't be like that cause I knew what was going on and i told and it tore my whole family apart. Sucked.