If there wasn't a traditional sport, I don't think that anyone will come up with the idea of fantasy sport because you must have the knowledge of sport you are participating in.
That is right obviously but times are changing and there are people who were not massive fan of sports in the first place but wanted to try out fantasy sports which led them to watching sports eventually.
It's a different thing entirely from gambling because you don't stake funds on it since the outcome depends on the best player.
Some may argue since some fantasy sports events have prize pools depending on who organized it so can be considered type of gambling. Do you have to watch a match to know who the best player is when you can just look at his stats in fantasy athlete.