It is popular in PC FPS gaming to have a smaller keyboard so you can go more distance to the left with your mouse hand before running out of space. Have you never hit your keyboard with your mouse? More likely to happen when playing with lower DPI/sensitivity. I don't use numpad for anything while gaming so it's extra space I'm happy to get rid of. All these people saying they don't get why someone would want a smaller keyboard are probably confused because they don't play FPS or games that require mouse space beyond their 6 inch mousepad.

Some gamers even position their keyboards at very harsh angles or nigh perpendicular to their body all just to make more space.