I don't think the demand was really there, although that may be changing. The PC has always had access to arguably more accurate racing sims. Forza and GT fall into the "if it feels real, it must be real" category in spite of the fact that the race physics are quite dumb compared to PC-centric race sims such as Race 07 just to name one. In racing, many gamers equate difficulty with realism which is silly.
I think what you are asking for is all the fluff that comes with games like Forza and GT that is more about busy work and collecting than actual racing...like painting cars and showcases. While I think everyone would enjoy driving their favorite cars in these games, I've never bought into the aspect of having so many licensed cars because the list of cars that are actually competitive in most circumstances in game tends to be quite small.
With racing games, less is more when it comes to realism. We tend to see the most realistic modeling when the develop has few or one car type to replicate. 500 licensed cars means inflating the budget and diluting the time spent on each car. Forza might have fewer cars than GT, but they have a solid release every two years, GT has a much more sparse release pattern that has recently been plagued by underwhelming delivery. Whats the point of having 500 cars when only a fraction of them are modeled top to bottom, inside and out. Including hundreds of PS2/PS3 era "standard" cars and calling it good is hardly awe inspiring.