Your thread is incredibly loaded OP, you clearly would not be open to anybody's reasons.
For what it's worth our reasons are as follows
A) the vast majority of our holidays have been paid for/with my parents. You have said her family "should" include them but this is not on any level your place to say. We do not claim to be "one big family" to the extent that my family is also my DSS's family. He is DPs child, my DSS, my DC's siblings and that's as far as it goes, there is a natural acceptance from all sides that he is not related to my family and doesn't have much to do with them, as my DC does not with his maternal family. There is more than one way of having a blended family, your view is not the only acceptable way.
B) Any holidays we have gone on and paid for ourselves (just the one I believe) we could only afford as it was during term time and DSS could not have gone. This was a cheap holiday btw, before you say we could have gone on cheaper UK holidays during the holidays. No, we couldn't.
C) Materialistically, DSS gets much more luxuries than our DC and does go on trips with his mum. I disagree with you that it is all about the quality time and not the actual experience. Of course that's nice but the experience itself is the main factor in my view. They have quality time at home.
D) Like it or not, my DP has chosen to have a new partner and does have a duty to make that work. I do not actually enjoy trips we take with DSS for various reasons and would not be willing to only ever have such trips, though of course I do suck it up for some.That is just the reality of it I'm afraid.
We are more practical people than many on here and don't really buy into the outrage about how a parent could enjoy a holiday with "only one of their biological children". My DSS is absent the vast majority of the time from the vast majority of things we do due to living with his mum. My DP is well accustomed to enjoying himself whether he's there or not. It's a different experience to living with both biological kids.