How could they if it's their first experience of being a step parent or having a blended family?
"You went into this with your eyes wide open"
Yes indeed, full of good intentions and hopes for the future in most cases.
Nobody can predict how dynamics will play out later on down the line.
I don't think any step parent enters into the role, choosing to settle down with somebody who has a first family, in full knowledge that they will be miserable or not cut out for it. What a ridiculous thing to say.
Please stop saying this to step parents who are finding it difficult.