When you create a game, you want the game to work with everybody, and not everybody has a monitor that can go above 60hz.
Just like CattleRustler said, the human eye can't tell the difference anywhere above 30 fps.
1-4 fps - MAJOR lag
5-7 fps - lag but still can get around
8-16 fps - good quality
17-24 - almost perfect
24-30 - no lag at all (meets "standards")
30+ - exceeds "standards" (in other words perfect)
Most games like to keep their fps a steady 30 (based on what I've played) because any monitor you find nowadays support 30+ fps. 30 frames per second is the closest you can get without experiencing some sort of lag.
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