Notebooks sacrifice processing power and memory for portability. This means games won't reach their maximum potential (lower framerates and graphics settings). Video editing requires a lot of memory.
Supercomputers are usually tailored for very specific tasks and no good for gaming or general video editing (they do however excel at rendering 3D scenes).
Tablets sacrifice processing power and memory for portability but also trade the mouse and keyboard for a touchscreen. They don't have the memory for video editing and dedicated gamers usually prefer a mouse and keyboard.
This leaves the desktop PC as their preferred choice which has room for lots of memory, a fast processor and a fast graphics card.
