Just a wild guess, but maybe because its a sim? One thing that sim racing communities (which would then include the wider community around GT5) tends to look for is a more realistic sim.
As I said before, you want more realism? Get rid of the heaps of awful drivers on GT5, before changing anything...
Which would be great if the tyres acted like a really bad real world tyre compound, but they don't, they don't act as tyres should do at all. I also fail to see why developing a better physics engine would stop us getting different/more cars and tracks? That would imply that the same people have to work on all of these element, which quite clearly is not the case.
Because, believe it or not, even PD works on a budget. And I doubt, when they won't be able to do something for GT5, they actually manage to pull it off for GT6, PLUS give us more tracks and stuff. This would mean a double increase in workforces really. (If you get what I mean.)
What an utterly ridiculous thing to say. I work in the motor industry, doesn't mean I have instant access to a wide range of cars and tracks to use at my personal whim. Very, very few people do, as such we tend to use sims to allow us to explore and enjoy what we can't always sample in reality.
I know, very few people do have access to a lot of different cars, and high speed track racing / drifting. Thing is, you can improve the game all you want, but in the end it's not the game that makes me come back, it's an awesome community.
For example, Decoy, Lock2Lock, Blueninja, Marbel, ... All these guys are great people, and know how to handle a car. If they weren't there, I wouldn't play GT5.
Improve the player base, improve the game. (Much more than physics would.) I accept that GT5 doesn't have perfect physics, and I don't mind because the drifting, believe it or not, is ****ing awesome nonetheless.