Yeah, let's start a discussion with conspiracy theories about why a brand wouldn't want their cars on the highest selling racing game...
It's not conspiracies, it's a debate. And in a debate, you need a side with the rose-tinted glasses off, that sees different options and has different opinions. That's what keeps the conversation alive.
Highest selling racing game or not, they've only released 2 games in the last 7 years, and nothing on the current gen... Why would Porsche's PR people consider giving the rights to Polyphony Digital first if they can't be promised their assets are going to be used in a timely and correct matter?
And, how can an LMP like the 919, that was built to follow a very strict and specific set of rules from a real racing organisation even stand a chance of competing against the VGT cars we see in the footage going against 908s and R18s, that are built out of the minds of engineers and designers who were told to create a car with technologies that do not even exist for a series that does not exist either? Where does a turbocharged V4 with an electric motor (I'm dumbing this down here) compare with a laser pulse motor, or a hydrogen fuel cell, or pneumatic cylinders (which doesn't actually make sense) and a 7L V10 that somehow makes the car weigh less than the current LMPs? Unless Porsche "builds" a VGT car, but then with PD's quick-time integration, that would get added to the game about 2-3 years later, and then we're back to square one. The very basis of the VGT vs LMP Gr.1 in GTS is flawed, but that's another story for another thread.
My point is, these are all details that might not be important to the PR people at say, Hyundai, Mazda or Audi, but given Porsche's history, it might be something that ticks them off. Again, this is speculation, but it's something to think about before going off on PD or before hoping too much... I can't imagine Ferrari being too happy about a 458/488GT3 virtually competing with a VW Golf built to similar specs.