Because some art is an investment grade asset driven by the psychologogy of scarcity creating "value", and a marketplace with enough buyers for whom having enough money is a small concern. Spending $1,000,000 to get $2,000,000 five years later for some dried paint doesn't seem irrational, though perhaps inexplicable. Though there does not seem to be a correlation between complexity and simplicity and the price of art, unlike some "modern" linestages.