When someone bumps into you and you fall down in excruciating pain, shouldn't you leave the game? Happens in other sports all the time. In baseball, football, basketball, hockey, tennis, golf or any other sport I can think of here, if you are in that much pain and agony, you will be assisted off the field, either carried off or on a streatcher.
You are missing the point. In any of these sports you just mentioned, if there was a chance you could eliminate the other player by exagerating how bad you got hurt, you should see the same type of flopping.
Want another sport where you see this? Basketball. Why? Because if you can convince the referree that if the foul was a serious one, you will hurt the other team with technical fouls, flagrant fouls, etc etc.
Another thing that you need to understand is that if a player flops, then realizes the ref will not give a yellow card and just gets up as if it was nothing, he can get a yellow card for trying to deceive the ref. So once you go down, you stay down for a bit to save face.
Despite this , it is still a great sport.