No kidding. The question is why. Why is that painting worth $142 million to anyone???
Why is any nonessential food, shelter, or clothing item worth anything to anyone? In economics, they use the utility or enjoyment a person gets from a good as a measurement of value. Everyone converts their individual utility into currency at their own internal rate, meaning you might value a reproduction of the Kramer painting from Seinfeld much less highly than I do, and there's no sense asking me why I'd pay $20 for it when you only think it's worth $10.
Whether an individual collector's reason for valuing the Bacon painting at $142mil is because it's Bacon's descendent, or someone who prefers that style above all else, or because he knows his neighbor is only willing to pay 140mil for it is immaterial.