Just cobbling on features without making the game a consistent quality is exactly what they're trying to avoid. It would hurt to "just add it". Because if they add it just as a small side feature just to add it as a bullet point, people will complain that they did that. If they throw a half baked feature into the core game and make it something that can't be avoided people will complain about that as well. If they can't do it to a high standard, they're better off not doing it until they can.
If Forza's lighting engine is really as complex as it's hyped up to be, then having upwards of 32 points of constantly moving light, along with a single massive one, could be a lot more complex than you'd think. Especially since Forza's got pretty heavily detailed reflections of cars on other cars, meaning they would also need to find a way to get the headlight reflection to look good. They'd also have to worry about illuminating the interior if they wanted to make it look right, so there's more to be considered than just tacking some headlights onto them and killing the sun.
As far as I've seen the only game that ever got close to really nailing the way headlights interact with other cars on the track are the Dirt and Grid series, which both have the same lighting engine. And neither have the physics complexity or car detail of Forza.