This phenomenon only happens when there is fuel in the exhaust that hasn't been burnt in the cylinder. Running nitromethane will do this too. However, the developers are working on important things, like physics engines and pretty new cars. Exhaust flare is not pertinent compared to these.
You're right, it does happen when fuel hasn't been burnt in the cylinder. That's how an anti-lag system (ALS) works. We all want the physics bugs to be worked out (that goes without saying), but if/when the time comes adding ALS would be a good performance add-on for making a tuner/WRC setup. Plus, cars that spit fire are awesome.
It would be amazing if we had anti-lag but then people (like me) would complain that there are not rotaries to use it with. 4 and 6 cylinders are ok but rotaries with anti-lag, that would be f****** amazing. Hope you like the mike tyson meme