Thats because everyone one at Nintendo is on drugs, the CEO's have planted aerosolised Crystal Meth in the airconditioners its great.
Here is my Rundown of the Legendaries so far
RBY/FRLG: 5
Articuno: Part of the Generic trio
Moltres: Part of the Generic trio
Zapdos: Part of the Generic trio
Mewtwo: Mans Attempt at God
Mew: Effectively God
GSC: 6
Suicune: Part of the Generic trio
Entei: Part of the Generic trio
Raikou: Part of the Generic trio
Lugia: Pokemonification of Night
Ho-oh: Pokemonification of Day
Celebi: Pokemonification of Time
RSE: 9
Latios: Guardian?
Latias: Guardian?
Kyogre: Pokemonification of the Water
Groudon: Pokemonification of the Earth
Rayquaza: Pokemonification of the Air
Regirock: Part of the Generic trio
Regice: Part of the Generic trio
Registeel: Part of the Generic trio
Deoxys: Mans second attempt at god?
DPP: 12
Mesprit: Part of the Generic trio
Uxie: Part of the Generic trio
Azelf: Part of the Generic trio
Dialga: Pokemonification of Time
Palkia: Pokemonification of Space
Darkrai: Pokemonification of Darkness
Giratina: Pokemonification of Dimensions
Heatran: Pokemonification of Natural disasters?
Cressilia: Pokemonification of the Moon?
Regigigas: Ruler of 3 Regi's
Manaphy: Pokemonification of the Heart?
Arceus: God?
So in generation 7 if it goes on for that long we can expect 20+ legendaries
How about a Life/Death combo. Grass/Dark maybe
Technology vs Nature
Good vs Evil
2 pokemon into 1
3 pokemon into 1
Another lame trio of pokemon maybe bugs this time
Thats all I can think of
~CEB