Did Porsche have a special deal with Turn 10? I had no idea GT hadn't got the Porsche license. Anyway, lol at Turn 10. I love Forza 3 but some of their shit lately is straight bad business.
It's exceedingly unlikely that any car manufacturer would enter into any kind of exclusive agreement with any car game, regardless of the game's dominance within the market. Car companies are just insanely big, leviathans of the business world, and they have too many other licensing issues to consider.
For example, if Ford pays for product placement in the James Bond movie to have him drive their latest model, that becomes an advertising interest to them. If the licensed game-of-the-movie includes a driving sequence from the movie where Bond was driving that Ford, that car would need to be there as well; but if Ford had made an exclusive agreement to have Ford only appear in Forza that year, things become uncecessarily complicated.
I've also found it very, well, unfair in that "product placement" and "licensed appearance" are really the same thing, but they're exactly opposite in terms of payment: the weaker party is paying for presence. Ford must have paid serious money to have Bond drive that Ford in
Casino Royale, but if Schmodunk Racing's
S1GP wants to use Ford in their game, the publisher or developer is going to pay Ford for that right, even if the racers actually competing in that league are driving Fords. Even if they're driving a Ford that has been so heavily modified that its only remaining factory parts are not even visible, the licensing agreement is still required.
Note: I'm only using Ford as an example; all car companies have largely similar policies.Hell, depending on how poorly constructed the initial contract is, it's possible that the game-of-the-movie Bond game would need to pay additional money to Ford to use the car's likeness, as well as other contractual guarantees that the car won't be shown rolling, on fire, crushed, or otherwise damaged exceedingly, because Ford is worried about their image in the game -- even if those scenes happen in the movie, though the use of the car in the movie is also constrained by those separate contractual agreements.