Exactly. GT originated with Racing Modified concepts such as the del Sol RM and GTO RM. All were completely plausible, realistic and generally based of an actual classification. It simulated something that was actually theoretically possible, and were still down right rad.
Assetto Corsa has cars that don't exist, either - I've never heard of an F40 Step 2 in my life, but it's still realistic. I'm not sure where the racing game industry has gone to necessitate fantasy laser propelled cars in favour of mid engine race cars of cheap Japanese run-a-bouts.