For me I think a lot of it comes from the vastly different ideas people have of a "blended family" and lots of people refusing to see any way but their way as the "right" way.
I think some poster's (and it is often those who have no step children of their own as well) have this ideal image of a step parent who loves their SC like their own child, treats then exactly the same, becomes essentially a 3rd parent doing everything they would do for their own child for their SC, missing them when they are gone and so on...
I'm sure that can happen but I think for most people it just isn't like that.
There is no one on this earth that I could love like I love my son. My step children don't even come close I'm afraid. I'm not sorry for that. He is the absolute love of my life and my whole world, no one competes. And I truly don't believe that makes me unkind or cold or bitter or any of the other things people say. I'm still very nice to my SC, I still like and care for them but it's just not remotely the same level. I don't love them like my own DC and I don't miss them.
To me the perfect blended family is one in which the SP is not put upon, is not used as childcare, is not expected to pick up things the parents should be doing. Me existing is not an excuse for my SC's parents not to parent. I don't expect to be needed for school runs or holiday cover pr anything like that. I'll help in a pinch and if I'm free like another family member may do but I would loathe the idea of it being expected.
And I'd feel the same if my son ever had a SP. No other woman or man will ever be responsible for my son. He is mine and his father's.
I hate this whole thing on here as well of you not being able to do anything with your own DC without always thinking about or factoring in DSC. Always analysing everything you do and whether it is potentially exclusionary etc... I could not be bothered with that at all. I will continue to parent my son how I want to, I will continue to do things with him as and when we want to and I won't feel guilty about it. I'll never feel guilty for simply being a mother to my son. Not ever. I'll never feel guilty for doing the same things for and feeling about my son as my SC's mother does and feels for them. He is my priority, he occupies most of my thoughts, my SC do not take up the same headspace and never will just like I'm sure they are and do tor their own mother. It is not important enough to me to analyse everything I do and fret so much about being exclusionary, to take away from my own son.
I think people hate that SMs don't feel the same way about SC as their own. I think it worries some mothers on here when they read it and that may be fair and understandable, but I also think it's unrealistic and if most were being honest they couldn't and wouldn't either. It's an unrealistic ideal that rarely works in practice and I think the pressure of it is what causes the resentment and bitterness not the actual "not loving like your own" itself.