Art is considered so valuable because it is something that is beautiful, and beauty is something hard to capture. Art theft is such a costly crime since art is meant for everyone to enjoy, and stealing it and selling it for profit only limits that joy to only one person. Which makes the art truly worthless.
There is a lot being done to curtail the theft of art, with the speed technology is growing there are machines that protect it, or even make a copy of the art while the real one can be protected.
Hope this helps!
