Question

why graphic card is more sufficient for playing games on pc

Answer 1

Because RAM couldn't had too data in real time interaction like actions on keyboard and mouse and animation at same movement.
So do deal with animation and audio video other memory instructions graphic card required. BACIC GAMES AROUND 2 GB WOULD BE ABLE RUN WITHOUT GRAPHIC CARD BUT LARGE GAME NEED GRAPHIC CARD.

Answer 2

Graphic Card are built to handle gaming performance only so they are more sufficient for playing games in PC.