They use it specifically to denote that the object was formally accessioned into the collection. They use it because selling accessioned items has legal and ethical ramifications. Museums also own objects that have never been accessioned. When those are sold they are simply sold. There is a huge difference.
And trust me. No one, let alone the millionaires you believe are out there, will donate a penny to a museum that doesn’t live up to it’s legal and ethical obligations. Nor should they.