I sometimes think it's because a lot of people are unable to put themselves in someone else's position.
For example, with mil, some forget that, for example, dh is the mum's child, just in the same way that our child is ours or that we are our mother's child. It's seen as a position of 'other' somehow.
The same with step parents, we forget that our child is very much a part of their life in many cases and the step parent's life and that of their children is no less important than our own or our children's.
How would I feel as a MiL who has loved, supported, guided and cherished my children to be sidelined by my son and my son's wife (I only have ds not dd) as though I suddenly don't matter as much as her mum or that I'm an inconvenience or interfering? It must be hard to move from being a mum to being a MiL and navigate the relationships successfully.
As stepmum (I am one), I've been very, very lucky in that dsd's mum is very supportive, very encouraging of our relationship and never ever resentful of the bond that dsd and I have formed over many years. I know anecdotally though, that this is unusual. I don't know how I would have felt to be micro-managed or criticised for my (always well intentioned) efforts with dsd.
If I wrote about mine and dad's relationship on here, I'm sure I would get criticised for "over-stepping" but in our family (which included dsd's mum) I am absolutely confident that I do not. Some people would be totally unable to see that.