Because it's not actually that simple.
Fanatec has no official PS4 license. Instead, it has written PS4 compatible drivers, but they have to be embedded into games by the game vendor themselves.
As of now, the only games on PS4 which have these embedded drivers are Project CARS and Codemasters F1 2015.
http://www.projectcarsgame.com/home/fanatec-wheel-support-comes-to-consoles
http://blog.codemasters.com/f1/06/f1-2015-supported-wheels-and-controllers/
Games like The Crew, Driveclub and WRC 5 do not support Fanatec wheels.
The fact that Driveclub doesn't support Fanatec at the moment gives us all pause for concern, since Driveclub is first party Sony. Gran Turismo, being also first party Sony, may or may not be willing to embed Fanatec drivers. Only time will tell.