Because cmd.exe is a fixed width based on the character count. You can drag the height down but not the width sideways. If you right click on the black C:\ icon in the top left and go to properties, the layout tab will let you set the character count width and in turn change that, or alternatively set the font size which will scale the window to suit the character count at the new font size.
You might have been facetious to prove a point but surprisingly few know you can change those settings there, and I like making my terminal oldschool-hacker-green :)
edit: Actually, I've no idea what I've done but now I can drag the window arbitrarily and it'll just show more characters. Maybe I set a font and now can do it? I don't know!
edit 2: I made it go to 160 characters, then set it back to 80, and now it'll let me drag it out to 160 but no more? This is so confusing Microsoft is on drugs
edit 3: Oh, ok. Screen buffer size sets the 'you can resize to this maximum' whereas screen size sets the 'you start at this size'. I'll stop making edits now.