First, let me preface this by saying I'm not an expert, just a redditor who's read a lot of articles, so I might be off the mark a little. Feel free to correct me if I am.

How come some people don't remember their dreams at all? I dream every night and can vividly remember them.

I think there are at least two things to consider first when trying to answer that question:
- Why do some people have vivid dreams, and others don't ?
- Why do we dream ?

When considering the first question "Why do some people have vivid dreams ?", keep in mind that some people don't visualize thought at all, and therefore don't have vivid pictorial thoughts, let alone dreams. If someone asked you to picture the face of a friend or family member, could you ? Not everyone can. I couldn't describe my family's faces if their lives depended on it. I just can't picture them. How about if asked to picture a house or a tree or an animal and describe it. How detailed are those mental images ? Some people simply aren't able to picture things in their head, or the pictures are dark and faint approximations of the objects. Their dreams are less likely to contain rich visual information, and possibly harder to remember because of it.

The second question from above "Why do we dream ?", might have an explanation in a recent theory of why we sleep. One of the many processes that occurs while we sleep is the purging of neurotoxins from our cerebrospinal fluid. The blood-brain barrier prevents many toxic substances in the blood from reaching the brain, but that also means that the brain can't flush out toxic metabolic byproducts from normal cellular activity through the barrier either. A separate glymphatic system is used to clean the brain by circulating cerebrospinal fluid using a network of microscopic glial channels. When we sleep, the glial channels expand by 60% to allow for the efficient circulation of cerebrospinal fluid throughout the brain, which washes away the toxins that have built up throughout the day. When the glial channels expand, the increased separation of neurons disrupts the their communications, making it difficult to process sensory information, and presumably prevents coherent thought. This process cycles with our REM sleep, and as soon as we wake up, the process shuts down entirely, the glial channels constrict, and the gaps between neurons close to their normal states.

I would speculate that we are groggy when we wake up because the gaps between neurons haven't recovered fully, and the neuron signaling is partially disrupted. I would also speculate that we dream because of the neuron signal disruption. Basically, our normal through process dissolves away as the neurons spread out, resulting in fragmented and nonsensical narratives and imagery from partially functioning memory recall and cognition. The wiring in our brains are effectively changed during this process making a temporary mess out of our minds.

So why do some people dream vividly and remember their dreams, and others don't ? This could be heavily influenced by each person's glymphatic system and how much disruption occurs in neuron signaling when the glial channels expand. It could also be heavily influenced by the nature of each person's normal through process, and whether they are visual thinkers, or not.