Because it's a really tough job sometimes?
I am a parent, step mother and step daughter myself.
It is not always easy to love other people's children, to get on with ex wives/husband's or to sit on the side whilst your spouse parents with someone else, to know when to stay out and when to get involved, to be fair and respectful, kind and welcoming whilst still having a say in your home.
There are days when it isn't easy to be and do the above, when the children are testing or I don't agree with DH or his ex etc... but I do it and thankfully have a really lovely little relationship with the kids because of it.
My step father is also brilliant. My Dad was not absent in any way and I'm close to him beyond measure but I really believe my life has been genuinely more positive by also having my step father in it.
Guess I just wanted to spread a little appreciation to those of you who, like me (if I do say so myself), are doing damn well at what can sometimes be a thankless, uneasy, difficult but often lovely job!