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.
On windows 11 I’m having a hard time finding apps that it doesn’t work with. Firefox, paint.net, inkscape, audacity… all work. And every single application made by microsoft also works