Try it. Move your Steam window around. See how it snaps to the edges of your display(s)?
Why is Steam the only program (I know) that does that natively? I would love for ALL my Windows programs to do that (without installing 3rd party softwares/scripting).
EDIT : I’m on Win10.
All my windows snap like that, I don’t know what you’re talking about. Just go to System Settings, search for snap, go to Window Behavior, click on Movement tab at the top. You can configure your window snapping preferences here.
On Windows10?
No, I don’t use Windows. Steam is built using qt which my whole desktop is built with, the qt devs like snapping windows and I like them too :)