Those speaking of spaceships have never heard of LFS, which is what GT5 would be (minus the car setup options/detail), as pointed by another user, if it had only fantasy cars.
That LFS doesn't have real cars, doesn't make it less realistic in physics and driving behavior (in fact it's better than GT5 in several aspects).
Anyway, such GT would probably have less cars than we're used to, because creating and modeling original (=that are not slight varations of real cars with badges taken off) and believable fantasy "generic" models (GT isn't just about "exxxtremez" supercars) completely from scratch and imagination is way more difficult than modeling real cars from reference (photos, scale models, etc). So, given the same amount of selectable cars (3-400?) if would probably cost PD the same as acquiring licences and modeling real ones, if not more (factoring designing and modeling time).