I think lots of things are at play.
Firstly, men of certain dispositions and attitudes are more likely to seek a replacement wife to take over widely duties (including anything to do with his children).
These men are a bloody nightmare to live with under any circumstances. More so when they aren’t your children.
People in these situations are disproportionately going to be posting on MN. The more liveable end of the stepfamily spectrum doesn’t generate the issues people post about.
Then there are loads of things caught up in the dreadful responses:
General misogyny. Projections from disgruntled ex wives and former stepchildren and such like.
But also an incredible slight of hand happens where people seem utterly unable to see beyond the evil SM/poor stepchild narrative. The actual causes of the SM being overwhelmed and even resentful are totally hidden behind this and people do not want to see them.
It’s the same shutting down of critical thinking whenever the figure of the vulnerable child is evoked that is deployed to shut down debate and critical thinking in so many areas. It’s an extension of the issues around ‘child saving’ campaigns generally. They are incredibly effective at displacing and oversimplifying complex issues.
Show people an image of an unhappy child (the younger and cuter the better) and it seems to prevent them from looking at the situation critically - even where they would usually do so in situations not involving children. People focus on all the wrong solutions - identifying a culprit to be vilified, throwing money at the symptoms, general handwringing - and ignore the real causes.
So the overwhelmed stepmum is berated and told she’s emotionally damaging the children etc, and people literally do not want to look at what the child’s parents are doing. They get angry at anyone who tries to get them to and make all sorts of excuses even.
All so very toxic.