Weird. Mine stay. But I’m on a notchless MacBook Pro. Maybe it always leaves room for the menu bar on the mother machines.
Probably the reason Apple makes it that way is because they don’t want the notch to intrude into the content area of a window. It makes complete sense.
May I ask what you’re trying to accomplish? For what purpose are hiding the menu and hoping to drag the window up? That’s essentially what full screen is. Click the green button at the top left of the window and see if that works for you.