Why? Cars are only really a part of what makes GT/FM. There's the included tracks, features, community features and a whole host of other things to consider. People were going crazy when they saw GT5's weather/night trailer, and that had nothing to do with the car count...
Like yourself and SavageEvil said, GT6 really needs a livery editor/decal feature. For me that's crucial, and if GT6 doesn't have it, then I may very well move over to Forza, regardless of respective car counts. But this isn't a livery editor thread, so I'd probably better just leave it there...