Emotional Denial

Why do people stay in abusive relationships? I have a friend who is recovering from an emotionally abusive relationship and another friend who is trapped in her own. The second friend is in the state of denial that anything is wrong even though it is blatantly obvious to the rest of us. The first friend is thankful that she can now recognize the signs of one, although at the time she also was in denial.
I was googling (is that really a word?) emotionally abusive relationships. This is a definition I found for it by Beverly Engel, therapist and author of The Emotionally Abusive Relationship, "any non-physical behavior designed to control, intimidate, subjugate, demean, punish, or isolate." In 2000, there was an estimated 8 million people being emotionally or verbally abused. 8 MILLION! That is unacceptable.
I understand that it is hard for people to see when they are in an abusive relationship because they have been so brainwashed or are scared of their partner. I think that is when it is important to have a solid support system. Even if the abused won't listen, I think friends and family members need to try their hardest to help them and love them through it.
I have a cousin who divorced her husband almost two years ago, after just over a year of marriage. He was so horribly verbally abusive that it seemed unreal when we found out. He made comments on the tire around her waist (which she lacked in reality) and how she "had better not get fat like her sister." He also made her pay for her own groceries, expected her to do all the cooking and cleaning, buy the things for the house, and do all this while she did not have a job. They lived in the middle of nowhere and while she looked for work, could not find any in her area. In his anger he threw one of their dogs against a wall and tied another up to the back of his 4-wheeler and dragged him around their yard. This was unknown to all of us until she filed for divorce because she was scared to say anything while they were married.
I have never really been in an abusive relationship, so I don't know what it's like or how hard it is to get out of one. My heart breaks to see all these people, men and women alike, become a shadow of the strong person they once were. They lose their ability to think, their voice, their self-esteem and confidence. They brag up their parter as if he or she is the best, smartest, most wonderful person in the world to try to change everyone's perceptive.
I feel sorry for these people. I know, through experience, that even if you try to show someone they are in an emotionally abusive relationship, they aren't always ready to accept it. I guess we just have to keep loving them through it.