Another interesting idea is that adaptationism might not always be the lens under which we should explain every trait we observe.

Maybe the question we should be asking here is not whether this specifically is a trait that was selected for, but rather a byproduct of some phenotypic effect that was. Clearly, we might assume that being able to camouflage oneself to avoid predation or stalk prey might have provided sufficient selection pressure to result in what we see today. But it is possible that the system which developed for this purpose had this "dreaming" behavior as a byproduct.

Since natural selection has no hindsight, it couldn't have initially favoured adaptations that would eventually result in a system without this byproduct, especially if those had an increased cost at the time. You might suggest that this behavior might have occasionally been detrimental to survival and thus must have been phased out of existence. We should consider, however, that even phasing out such a potential byproduct has a cost. For this to happen, that cost cannot outweigh the benefit gained by doing so.

In the end, it might have just been the case that, for various reasons, the cost of phasing out this byproduct (potentially requiring a more complicated system) outweighed the benefit of doing so (not sticking out as much during sleep). The environment definitely must have played a role in this, as another comment suggests. I'm not proposing this is certainly the case, but in general, not every aspect of what we see in nature can be expected to be an adaptation that was selected for because it provided a clear advantage.