Why is art so valuable and expensive?

Because some people can see a hidden meaning behind it that they cherish

Art is so expensive because it is entirely unique, and not everyone can do it. It requires a huge imagination to be able to create professional art.

Another reason is because of the medium used. If a certain piece of art was made with, let's say gold, then it will be expensive for two reasons. One; the creativity, and two; the medium used.

Sometimes it's expensive due to the fact that the artist makes very limited artwork. They may not make so much art.
The technique also makes it pricey. Suppose a person painted with their feet, and it was a professional piece. It would be expensive. 

If art was made with unique tools, like bread, then it would also be expensive. A sculpture sculpted with bread is pretty strange, right? Not to mention unique. Artwork like that actually does exist.

Have a nice day! :)