Because window backgrounds were gray in NeXT, because buttons used both white and black to create beveled "3d" borders, because beveled buttons were a fancy upg

Because window backgrounds were gray in NeXT, because buttons used both white and black to create beveled "3d" borders, because beveled buttons were a fancy upgrade compared to simple borders in 1-bit graphics. Screen resolutions and drawing speed weren't good enough to waste on thicker borders, so 1px lines had to stand out on their own.