No it's because we know we don't have a snowball's chance in hell of getting these cars. An FIA license to put these cars in the game is stupid expensive. PD's profits would tank. Yes Forza has it since they have money for things like that. PD uses their money for a more realistic game which I'd rather have.
Content for the current season is part of Codemasters licencing agreement so it shall not appear anywhere else. Understandable when you consider what they pay.
Older cars are a possibility but I just don't think we'll see any in GT6.