If you look at the world as if it was running on a tech tree in a video game, making closed source technology only forces people to retrace your footsteps(re-research the tech, if you will) or worse, find a better alternative (leaving you and your creation in the dust). By open-sourcing, you effectively stake a flag with your name on it in the tech tree and incentivize people to use your creation as a foundation for future techs. Sure, people will be free to use copies of your work at will, but they still have to give credit to you, and won't be trying to do things differently.
And that's not even mentioning that open-source stuff generally has a PR-boost compared to closed-source. If you make something closed-source, you will always have to worry about the appearance of an open-source competitor. I have seen closed-source stuff be completely eclipsed by open-source alternatives once the latter reached a critical level of quality. I mean think about it: Unreal Engine 4 is open source, would anyone start a closed-source project to compete against it? All the best minds will naturally ask you, 'Why not just work on UE4?' Yes you can fork it, but the public will always turn to the original of the fork unless your fork achieves something really different in a good way.
So in a way, open-source is the most asshole-way you can do things cause it shuts down any idea of a copy/similar alternative to your work.