People struggling with mental health and no safe support network can struggle to identify clear red flags or blatant abuse. A lot of people experiencing this will end up in, return to, or hop from one abusive situation to another because of this. The trauma experienced can make it even more challenging to see or manage.

It's also possible that they do see the flags and abuse, but feel that despite the poor treatment it's better than whatever alternative they may have to suffer otherwise. Financial dependence or other society-navigating dependance also leads to this. Need medication to manage but you can't cover it alone or depend on your partners benefits? This can also be true for people with poor physical health who get trapped for similar reasons.