I believe it's a sense of superiority mixed with a sense of wanting trophies.
I always make sure my discoveries are properly credited and provided to the peopl they belong to. But my family ...
Let's just say I've been going through the family collection and trying to return as much as I can to where they belong. It was painful, since I grew up around that, there are memories. But it was the right thing to do.
Interestingly, some items have proven harder than others to return. In some cases I'm entering deals to loan artifacts to various museums while transferring ownership to their rightful holders.