Nicknamechange - But I still feel immensely guilty for typing these words. As if I have let him down and he's going to be disappointed in me all over again.
My view is that these people were the ADULTS in the relationship, any feelings of responsibility lands squarely on their shoulders. It was their role to protect and love, if they were incapable of that I don't see that as being my failing...nor do I feel I was/am a disappointment by saying 'Dad, you were crap'! I am confident in my assertion that you can love someone and not like their behaviour - it doesn't have to be all or nothing.
Ginebra 'Husband and Epiphanies' - There's the end of my marriage in a nutshell. He did have one when he caught witnessed MiL verbally emotionally damaging one of our DC, but eventually slid back into the family tram-lines of behaviour and began minimising it. I thought he would have one when I confronted a relative of his and he heard lie after lie after lie drip from their mouths (yes, he did know what they were saying was completely self-supporting lies!) but I knew my marriage was a goner when he sat there looking tearful, said nothing until he sympathised mid-conversation with the liar's upset at being asked to explain their frankly shocking and destructive behaviour (he actually apologised to them...!) He might have one in the future, but clearly cannot overcome his conditioning so they are not quite the wake-up calls the rest of us experience.
Reading back over thread I recognise some Toxic 'Axioms'.
Illness - yep tick, MIL and her barnstorming Migraines. Only cure is a lie-down in a darkened room.
Keeping up appearances - OH YES! But I was always aware of that one. Appearances are EVERYTHING in their family - the 'naice' family is not all that, some very dark stuff under that veneer of respectability.
AttilatheMeerkat 'Nacissists and gift buying' - Words cannot express how much that post hit home! In-Laws didn't even do Birthday cards before H and I got together, and any present ever given to Mil was met with a moue. Text from MiL to (my) Husband a few weeks ago mentions his Birthday present - it's still not appeared (and never will). Ditto Christmas present.
The only thing that doesn't resonate are the melt-downs and tantrums some described, it's all passive-aggressive here. All the better to play the victim, and all the harder impossible to counter.
To answer the original question "Why do people do it?" - I would suggest that, either they are mirroring aspects of their own upbringing (the vicious circle/their 'normality') or they had some specific incident/s in their childhood that damaged their emotional development. The end result is we suffer for their failings.