In the simplest terms possible, because most of the things you just said don't fit very well with how the rest of the game is played. It's almost as if snipers are "outside" FF shooting "in". Getting potshotted by a sniper hiding in an obscure corner is not exactly something that the victim will want to encourage.
Though, this is not a response to the claim that "any noob can be a good sniper", because that is 130% false. It DOES take real skill to hit targets in such a fast-paced game, but it also is a skill that is not looked kindly upon by the victims for the same reasons (the game prides itself on being fast-paced, having constant movement and interactive/two-sided combat, etc).
If you want to show your sniper skills to people that will appreciate it, however, you can join the
FF.PL 2v2 Sniper League.