Art is not a financial instrument and behaves badly when treated as one. It produces no income. It cannot be sold on a Tuesday because you need the money on Wednesday. It costs something every year merely to keep. Any account of art as an asset that skips those three facts is selling something.
What art has done, repeatedly and across centuries, is carry purchasing power through the periods when financial assets did not. It is not correlated to equity markets in any reliable way, because it is not priced by the same mechanism. And it is one of the very few assets whose value is partly a function of how well its owner kept the paperwork.
That last point is where the house earns its fee. Two identical canvases, same artist, same year, same condition — one with an unbroken ownership record and one with a decade missing — are not worth the same money. The difference is not aesthetic. It is documentary, and it is recoverable only by work done before the sale, not after.