I wouldn't mind sony throwing GT4 as a ps2 classic. I know it won't ever get an HD treatment, as it shouldn't. but wouldn't mind playing that again.
Actually try GT4 in progressive scan mode on PS2, it looks amazing, so I'd like to think that if, or when, it ever makes it to PSN, it will be in HD or at least the progressive scan resolution.
They would probably have to renew every license for every trademark/brand to release these games again.
That's why I would assume they aren't available.
GT2 works just fine running off disc on my PS3