Rumour. Fact is we don't know what EA/Turn10/Porsche had for an agreement.
... or can't do it for other reasons.
Exclusivity. Magical word.
Same word that prohibited Ferrari being on PlayStation systems during last-gen (their agreement with Microsoft) until Ferrari Licensing Department decided to forget the exclusivity in order to allow promotion of the brand.
Same word that allowed Bernie Ecclestone to fully convert policy of his licensing department from issuing multi-licenses to multi-licensers (pre-2000) to current situation where only one license-holder is choosen after license-pitch (post-2000). With Ferrari as notable exception (as we know it from GT5) due to their unique position in accordances of Concorde Agreement.
Same word that allowed people from Simraceway to sign an exclusive licensing deal with the McLaren group about exclusive licensing their entire historical catalogue, including all F1 cars, with exception of MP4-26 (because it is covered by FOAC license).
Same word that granted Microsoft rights to GTA IV expansion packs on first half year. Or COD expansion packs. Or Pagani Huayra being in the Shift 2 game for the first 6 months after release for EA.
Money and will to do something. Someone comes with money and invest and want his investment
protected. Those are
other reasons. Having Porsche as exclusive brand in their title is comparative advantage to Microsoft (Turn 10) over Sony (Polyphony). And be sure they will do whatever they can to keep that comparative advantage to themselves. Because that particular comparative advantage is very, very important *mindset asset*. And it also provides additional profits.
Money can buy many things, but contracts are contracts and long-term business relations are long-term business relations. Sometimes you can have all the money in the world but still couldn't buy something you like.
Or you don't find it feasible. Or whatever. But real-life is often much more complicated than you think it is.