Why would anyone stay in a mutually abusive relationship?

If it gets to the point where both are abusive towards each other, is there a point for that relationship to continue? One party is abusive, then the one is abusive back and so one, back and forth. The answer would be to already leave the first time a partner becomes abusive, not scoop down to their level.

Someone just broke up after 4 years of that abusive cycle. I've heard of those relationships where both are abusive and they still stay. Why?

Why would anyone stay in a mutually abusive relationship?
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