The reason is that ligaments are poorly vascularized. In other words, there aren't many blood vessels to provide nutrients for the ligaments. And without nutrients tissue repair is not possible. Oftentimes, ACL tears require a surgical graft.
Soccer goalies usually dive to their left or right during a penalty kick. That's despite the fact that the statistically-best option is to stand in the middle of the goal. So why don't they? Because there is a strong bias toward "doing something."