Because some people find it very annoying
How to Disable Aero Snap in Windows® 8
Starting with Windows 7, Microsoft added Aero Snap, a part of the Aero Desktop experience not found in Windows Vista's Aero Desktop. Aero Snap allows users to drag any window to the top, left, or right side of the screen to maximize the window in a specific way.
Dragging a window upwards will maximize the window to full screen, while dragging it left or right will maximize the window to half screen, depending on which side you drag it to. While useful for viewing two windows at once, some users find the Aero Snap feature to be a bit too aggressive, as it will maximize windows when the user didn't intend for them to.
Fortunately, Microsoft provides the ability to disable the Aero Snap feature without having to disable Aero Desktop altogether or create a complicated registry entry.
