For a starters, blended families make large age gaps much more likely, and this is something that is often drawn out significantly specifically for the SCs benefit, to allow them time to adjust. That means not only that it's harder to find activities that suit both, and that things quickly become unaffordable. The younger children do still deserve to have those opportunities, that the elder likely had when they were young.
Then there's what a poster mentioned earlier, about dynamics when SC being around being far more focused on just them than they would likely be in a conventional family. This is something I definitely find with my DSS, and seems common with children raised in two different households with two different sets of values. In some cases it goes beyond just that, and the SC refuse to coexist with other members of the family, such as on the honeymoon thread that is currently going on here. If there is not harmony between the half siblings, or between SC and step parent, then it may be a bit much to expect them to never holiday without them. Again, this is just a part of the nuanced, complex dynamics that come up in blended families.
And finally, I would say some view holidays as just for the kids, whereas some view them as for the adults, too. I would say the reason dad goes along too is because it's also his holiday, and that of his partner. Certainly my DP knew when he chose to be in a relationship with me that I wouldn't be taking our future DC on holiday's alone if he couldn't afford for DSS to come. Sometimes compromise is just necessary.
These are just some of the reasons, not an exhaustive list, but the point is, step families often create unique issues and needs compared to conventional families, and all we can do is look at our own situations, the people involved, and make the best judgment we can as to how to meet everybody's individual needs.
To clarify, I wouldn't advocate never taking the SC on holiday. Just that in some situations it may be ok to sometimes not include them.