Because logical slippage due to the vagaries of language is a decided risk.
Here you're mistaking the location of the dream. Dreams in *our* world, as *we* understand them have these properties. But again (and as I said in my other post) we're talking about another world that we have no reason to assume is not fundamentally different from this one (in fact we might, for many reasons that don't need belaboring here, and that are bound up with the very logic of the proposition in relation to what we understand about our world, have many reasons to assume the opposite—that it *is* fundamentally different from this one).
How does a "dream" behave in another reality in which *this* entire reality can *be* such a "dream?" Who knows. Nothing of what we understand about "dreams" as we know them in practical conception is remotely similar to what we mean when we talk about *our entire reality.*
How does a "computer simulation" behave in another reality in which *this* entire reality can *be* such a "computer simulation?" Who knows. Nothing of what we understand about "computer simulations" as we know them in practical conception is remotely similar to what we mean when we talk about *our entire reality.*
All we have to do to call the universe either a dream or a computer simulation is completely throw out any particular characteristics that are unique and empirically attributable to what we mean when we say "dream" or "computer simulation" as we are able to make use of these terms.
In other words, sure, this universe is a computer simulation or it's a dream...for certain values of "computer simulation" or "dream" that, if we were to accept them as valid, make the terms able to encapsulate *just about any phenomenon*.
This universe could also just be another reality's version of a "jumbo citrus fruit" or of an "Oscar awards ceremony," for the same reasons, and with the same level of practical or logical utility obtaining for these statements. For Slashdot purposes, I propose that we collaboratively write a paper on how this universe is just another encapsulating universe's version of a "Netcraft confirms it, Linux is dying!" press release.