I just tell them that those other kids aren't my kids and don't live in my house. If they would like to go live with the other people, then they can live by their rules and not mine.
This is why I would usually do but man sometimes they can be resilient to such lectures. I'm an adult and normally mine is last but it boggles me how strongly the society influence their lives.
My son knows the value of money I'm glad to say. He knows that I can't afford to get him all the latest gear and he knows that if he wants something badly enough, he has to save for it. Hopefully it will stand him in good stead later in life!
That is one of my favorite sayings that never managed to get old in our home with the kids growing up. I also remember a great deal of "I can't". Now that one always bothered me!
I would start off with a joke: "Because you have good parents". Then I would tell mine that every household has different rules for different reasons, and leave it at that.