The reason why art is tied to dreaming is because art operates just like our dreams do. If you think about how art tends to be presented, it is typically a combination of different symbols, gestures, or movements that provide a kind of textual meaning. It is up to the interpreter to gather the meaning from these various symbols in order to comprehend the true meaning being depicted by the artist.
In your dreams, your unconscious mind acts as the artist and your conscious mind is more of the interpreter. Similar to how we analyze art through different themes, motifs, and various techniques, event sequences, and images, this is also how we come closer to the meaning of our dreams below the surface.
In an art class I was taking in college a few years ago, my professor would often remind us, "The greatest art comes from the unconscious." He explained to us once that the interpretation of art extends beyond the realm of the artist's perspective. This is why we don’t interpret art just by virtue of what the artist has thought and said about a given piece he or she has made.
Similarly, in literature there is a concept called, "death of the author", which takes root in a similar argument about the unconscious mind and art as a whole. This concept teaches literary critics to avoid restricting their perspective of a given piece to authorial intent. The argument is that meaning can be found from far beyond the limits of such containers of interpretation. This is an interesting phenomenon in art because it appears that much of the art we have comes from the unconscious mind. Have you ever talked to an author a book they’ve written? They usually know less about their work than anyone else does.
This connects us to part of the reason why art is tied to dreaming. Just as our dreams create representations of meaning, which sometimes seem totally random, there is often a deeper and more unconscious meaning being expressed below the surface. In a way, when our conscious waking minds are analyzing our dreams, we are a lot like guests wandering through an art exhibit. The meaning is depicted in various and often creative ways, but it's just up to the interpreter to figure out what the artist was trying to convey.