Sony don't want new fans of GT1, they want new fans of the game they're actually selling. Allowing other people to make their own variants of Gran Turismo also means having their very valuable trademark attached to products completely out of their control. Imagine if someone used the source code to make a version of the game where you score points for running over kids or something, I seriously doubt they'd want that associated with the GT name. Even just a badly-modified version which crashed frequently would reflect poorly on their brand.
It's also not just about not hurting them - they're a business, they'll only do what makes money. Spending the money to dig out old source code and release it for the goodwill of what is realistically maybe a couple of hundred people is absolutely not worth it for them, even ignoring the issues above. It's also source code that they spent of a lot of money to create, so giving it away for free doesn't add up.
There's also the point that the source code is technically useless to the public, as the Sony SDKs required to build it are not freeware nor have they ever been released to the public. Even if they were, there is no way to run a modified game on a standard Playstation console. For a source release to have any real utility, Sony would need to be implicitly endorsing pirating their SDKs and bypassing the copy protection on their consoles or running their games on other companies' hardware, all of which are the direct opposite of what they want people to do.
All of this is why you don't normally see source code releases for games in general. It provides a long list of issues and liabilities for the company, and basically nothing in the way of tangible benefits. We're frankly lucky that Sony haven't issued any cease and desist demands against existing GT mods, never mind supporting them in any way.