Hi all just started playing game of thrones and was wondering why house tyrell doesn't exist. Don't know if I would play them but would like to see them.

They do exist - as a trait rather than a playable House. So Margaery, The Knight of Flowers, The Queen of Thorns, etc., do have cards in the game.

Thematically, "House Tyrell" doesn't exist as a playable House for the same reasom "House Tully" doesn't exist as a playable House; their machinations and activities to claim power exist (at least outwardly) within the claims of other Houses (Baratheon and Lannister for Tyrell, Stark for Tully).

Also, House Aryn is getting some decent stuff in the upcoming chapter packs.