Forza and GT have a lot of money behind them, it would be difficult for an independent developer to come up with the cash to pay for all the franchise licensing. Also, those are marquee games for those consoles to showcase the console's abilities, they might be losing money on them. Remember that much of the promo for Forza 5 was made with computers that were far more powerful than the XBone.
I am a person who likes cars and video games, in that order(as opposed to somebody who like video games that happen to have cars in them). The only place I'll ever own a Ferrari or McLaren is a video game. Generally speaking, I think a person who buys a racing or flight sim is going to be a little older than the average gamer, that market is probably smaller than the mainstream gamer. No matter what driving game it is, public multiplayer racing is pretty much a demo derby with twelve year olds, it may be a lost cause.
After Forza 4 and Horizon, I now have Project Cars and Dirt Rally. Still using an Xbox 360 controller, I find Project Cars pretty similar to Forza 4 without some cars and vehicle customization. I like Dirt Rally because it gives me the "feeling" of driving. It may not be as technically complex as other driving sims or semi-sims, but it's fun to drive. That's the most important thing for driving games as well as in real life driving.