How come condoms can "protect" against hiv but aren't enough to prevent pregnancy every time?

Define failure: Condoms prevent both HIV and pregnancy without about the same success. When condoms "fail, " resulting in either pregnancy or HIV transmission, it's usually because they're left in the drawer on the nightstand. This is termed "condom failure" in people who list condoms as their form of birth control, but it really shouldn't blamed on the condom. The same is true for HIV prevention.