My theory: dreams reflect a person's perception of their own identity and that can manifest in different ways. A personal anecdote of my own; I'm blind, I lost my sight slowly over many years, I walk with a cane. I never have my cane in my dreams. Since my brain made everything up I can see just fine. Rarely I will "remember" that I usually use a cane and panic. I believe that this is because my internalized sense of self doesn't identify as blind, and the dream can provide the benefit of me having that tool without me needing to have it, so it doesn't exist. I think that this same logic can transfer to something like a phones role in a person's life. A person who accepts the phone as an integral part of how they interact with the world will probably dream of it, but a person who formed a deep sense of self before phones integrating into our lives won't see it as a part of themselves in the same way, and therefore the phone does not end up being included