Why married couple secretly cheat each other if you're not happy then file divorce isn't as simple as that why to cheat?

I have neighbor she cheated on her husband with me from last few weeks... her husband do night shift job she spent night with me. I asked her if you're not happy then leave him she told me I can't society will not accept me n he love me so much I was like wtf 😂😂
Why married couple secretly cheat each other if you're not happy then file divorce isn't as simple as that why to cheat?
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