Why people stay in abusive relationship?

Like my bfs ex broke his finger

They had fights with her attacking him amd he leaving her then she coming back crying amd he accepted her

Why he did that? why in general people this is love and its passion? how can this be passion and why people dont leave when other person hurts them?

Updates
6 mo
Also he was punching walls and i asked why
He said "to not punch her"
Is it even normal
Why people stay in abusive relationship?
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