Why do people stay in abusive relationships?

It is always online inland who are in the business of any public celebrity relationships as if they have shares in there. It is also the same online inlaws who are the first to notice issues in the relationship because of how closely they are invested in following the couples lives. This can be both good and bad because as much as it can be a little too nosey, they can help prevent cheating and promiscuity. One of the main issues that they also notice is abuse. Either through change of behaviour or vibe and mood. Their input and opiniosn are not always accepted which brings me to the question of why people would fight hard for a relationship when they are being abused physically or emotionally.