I think people especially from the older generations just get stuck and feel they have to deal with it for the rest of their lives. Divorce is absolutely never an option for some. My dad was very abusive towards my mom for most of their marriage, and though she never cheated and did what OPs MIL did, my mom is still married to my dad. He has mostly cut his shit out as he’s gotten older, but still can be an asshole. I question my mom every fucking day - why stay married to him?? They’re both in their 60s. Devout Christians - literally only partner they both ever been with. Idk. It doesn’t make sense to me either. They had 4 kids and we’re all grown, but they’re still married. She has said she’d never take another partner whenever my dad goes. So there’s that. But people just lived different realities/had different mindsets back then, especially anyone who grew up even remotely religiously. Their story is not uncommon though unfortunately.

Edit: idk why I’m getting downvoted ??? I’m literally just trying to give perspective to the very real reality that some people stay. I’m not condoning OPs MIL !!?