The problem with including a cool down lap is that, unlike the formation lap, there are people still racing and you wouldn't want cars that have finished the race to go and interfere with the stragglers that just about avoided being lapped. With the formation lap, you have control over your tyre temps and grid box position which directly affects your race start. But cooling the car and picking up rubber are not relevant to the game since your car isn't weighed by scrutineers, so there is no benefit to it. In most online races you get kids smashing their cars into walls as they cross the finish line as well. So you can imagine the mayhem if they were let loose on a cool down lap that served no purpose.
I'd love to see more realism and authenticity in the game, but there are many more features that I think would offer the player a better experience and value