I bet Hilbert did the same thing, and whoever he copied did the same. That's usually what turns out to be the case. I don't know that it's necessarily malicious; more minds are better than one, and the one who's most outgoing is probably the one who shares with the world and gets the credit. In the past when women and minorities were actually being oppressed, it was impossible for them to share any of their work and be taken seriously, and likely it would even get them punished. So they would have to find someone else who society allowed to do that.