You can never say one sport is harder than another.
Sure you can, that's a copout

Harder can be:
1. more work and time needed to reach a stage where an average player respects your game, even if you're not a pro yet
2. more work and time needed to be officially recognized as a pro, or just competitive with the best in the world
3. more work and time needed to meet the goals set forth in the game.. i.e. "par" in golf. Or if you like, to play as perfectly as the game permits. To hole-in-one a single hole in golf is the best you can possibly do for that hole. And it is harder than running a rack of 8, 9, 10 ball.
4. more physical effort, mental effort, and/or hand-eye coordination.
5. more variables to keep track of (equipment choice, wind, slope, lie, etc)
I mean, I get what you're saying, and every game develops standards and tiers of skill, and getting to the top of those tiers is hard work for everyone... but each sport puts physical and mental challenges on a person and it's fair to rate which have it worst. I think games where reflexes and speed (i.e. baseball) are required are harder than turn-based games, and games where physical size and strength are required are harder yet (basketball, football) because most people won't win the genetic lottery in those areas and need to work much harder to become a pro.