There's a reason.
People take GT as fact. The number of people who really think that the cars look, go, handle and sound like that, or that they can drive "this" on "that track" in the game they go on to replicate it in real life is... staggering. The disclaimer at the beginning of every game means nothing to them - they could hustle Jay Leno's Tank Car around Laguna Seca like a pro! (one111one!).
From a marketing standpoint this is brilliant
and a knife in the heart at the same time. Manufacturers want their awesome halo cars and best-selling models to be in the game, because they appeal to the next generation of car buyers before they even buy cars. The Skyline, for instance, would be nowhere near the fapicon it is without GT. They grab the hearts and minds of the future market.
But then you get manufacturers who are kinda precious about how their cars perform. Ferrari have always been notorious for it but they've apparently relented, to be replaced by Porsche. Remember how antsy Porsche were about the GT-R's Nuerburgring time? They went out and
bought a GT-R and tested it themselves to
prove it couldn't do the time stated - and of course PD and the GT-R are kinda inseperable (in fact Nissan in general).
And then there's the thorny issue of damage. If these people think a car looks, sounds, handles and goes in real life like it does in the game, they'll think it
crashes like it does in the game. Manufacturers get
very precious about how safe their cars seem to be and will
never allow the passenger cell to be compromised
even if it would in real life - never mind that the average GT crash would end with
any road car "ending up in pieces maybe an inch big". Ford will get annoyed if their cars seem to be easier to damage than Vauxhalls/Holdens/Chevrolets (depending on the market), Peugeot with Renault, Toyota with Honda and so on and so forth even, get this, if they
are in real life.
Ultimately, "real" damage won't ever be permitted in any game. "Realistic" damage won't be permitted either - cars will all be expected to deform at the same rate, except for manufacturers who aren't bothered (rare) or insist that their cars aren't so fragile and are then either excluded from the game or included because their inclusion results in good PR (Ferrari in Forza) - and in any case, real world individual vehicle damage testing doesn't stretch to a 140mph lateral nose plant on a crash barrier.
The
best we can hope for is semi-realistic damage, where damage severity is extrapolated, energy/angle dependant, not vehicle-specific and restricted to certain vehicles. It's easier with privateer race cars because the manufacturers don't even own the silhouette and you can apply whatever damage you wish.
It's pretty much GT being a victim of its own success - the more "real" it gets, the less comfortable some manufacturers are about it.
I have the retail version and its great... it will keep me busy til GT5 comes out... and even though I am getting Shift as well... I recommend Supercar Challenge to you all.... I cannot say the same about Shift because I have not played yet 
Still missing the torches and pitchforks though. 
