Also part of the problem is that art is fragile and showing say: a painting can degrade it, as light breaks up pigment. Even in storage, certainly paints/mediums change their physical characteristics. Many "hidden" paintings reveal themselves underneath the oils that make up the final painting as the medium becomes more transparent.
Maybe it should be more clear to the public if the idea that art isn't so much given to museums, rather that artwork is used as a speculative investment by a private party until such a time it makes financial sense to transfer it to a museum. There is also the fact that if one is powerful/influential, you can influence the value of the artwork just by owning it and keeping it out of circulation. The whole private buying and selling of the artwork from gallery owners to auction houses works to raise the value of the artwork through hype, really.
It can be a hell of a grift.
van Gogh wasn't very good at playing that game, even when his brother was an art dealer - that's such a leg up and he blew that advantage. van Gogh isn't known to be all that easy to work with. Attempted murder doesn't help with this. We know him know since we celebrate his mental illness.