But that's the thing. At the start, unless they're an evil sociopath, step parents do want to build a nice relationship with the children. And often there's a honeymoon phase where it all feels like it's working well.
However, over time, the children's parents take the piss. The step parent is guilted into becoming more and more obliged to do childcare, and has their good will eroded. Then the step parent starts to feel resentful and withdraws from the whole dynamic in order to protect their boundaries and their sanity. To anyone looking in from outside who has no clue about it, it can read as rejection of the children. But when you find that your partner and their ex have slowly taken over your free time and your free choice over how to spend that time, the only option is to take a hard line.
This is usually the point where the step parent posts on MN along the lines of the OP on this thread.
Those of us who've been through it know that there was likely a time at the beginning when OP felt very fondly for the DCs, but that quickly became leverage for her DH and ex his to take advantage of her independent wealth and her availability.
How else could OP start to change the status quo and get some control back without removing that leverage?
And ultimately none of this is of OP's making. The DH and his ex have pushed it too far and fucked it. Yet OP somehow gets the blame.