I think this is because the game classifies it as a tier 5 car when the career mode only uses cars up to tier 4. There may be another reason but this is all I can think of.
Yes indeed. I was also thinking why can't we use all those race cars in career mode. I don't get why they (the game developer) included those race cars but don't let us to alter their setups for different tracks.
on a side note, I don't understand why can't we modify cars from tier 4. I just hope that they would at least give us a chance to modify their appearance... for example, I'd like to add a GT wing onto the McLaren F1, just like how they did with the wallpaper.
yeah, I was attracted by that stunning wallpaper before the release of SHIFT, dreaming about driving that beast with that bodykit... but now end up only DREAMING.
I think this is because the game classifies it as a tier 5 car when the career mode only uses cars up to tier 4. There may be another reason but this is all I can think of.
I just hope that they would at least give us a chance to modify their appearance... for example, I'd like to add a GT wing onto the McLaren F1, just like how they did with the wallpaper. 
