I agree with you Spaghetti69, all throughout history mankind has been learning from it's mistakes, why can't PD do what the rest of mankind has done, still does and will do in the future.
It's an entirely valid point, and I doubt that they're being totally reckless in that regard. But it is sometimes important to go back and "reinvent the wheel",
sometimes. Ideally, you'd challenge every assumption at every point of the development process, which basically means research and experimentation - and that's potentially expensive, if only in time.
Gran Turismo was ground-breaking in its day, it was a "new" idea, rather than simply doing what had already gone before. I think the habits PD have established since then, to start everything with a blank slate, is something they don't really want to give up. I'd want to be the same, to be honest, but I am an idealist; being able to tailor things to your exact needs is an ideal situation if you can afford it, but I suppose there's nothing wrong with taking stuff "off-the-shelf" if it does what you need it to, although...
A recent example in racing games: the
iRacing guys tried to port their sound engine over to FMod, which practically "everyone" is using, and is an established "solution" based on a paradigm that has barely evolved in the last ten years or so. They decided that their own system was actually better for the job, because it already did stuff FMod couldn't, and FMod couldn't really offer anything new (for them, in their game) either without substantial modification - that's no slight on FMod, it just illustrates that when your game is so different, you just can't do what everyone else is doing.
Whether you succeed, of course, is a different matter.
I'm personally happy for PD to keep trying, and whilst I fully expect GT6 to miss the mark in some areas, it's clear they will get a lot right, too - hopefully a livery editor will be on the latter list!