Won’t they be more manoeuvrable in dogfights?
Fighters don't really need or want to fly at negative Gs for any sustained amount of time. There is no tactical reason to do so, and it is very uncomfortable on the human body.
A short duration negative G bunt may be employed effectively as a defensive maneuver, but overall a fighter will always keep positive Gs on the airplane. The body can tolerate many more positive Gs, which results in better turn performance, plus you can actually see in the direction you are turning!
Therefore it just makes sense to optimize the airfoil for best turn performance at positive G loading. Aerobatic aircraft spend a lot more time at negative Gs so it makes some sense to help them out with a more symmetrical shape.
What makes you think they don't. As the old saying goes "with enough thrust pigs fly just fine" To be serious, fighters also have to land and a little asymmetry would help at lower speeds, although I don't expect they use much, and it may only be a portion of the wing. The F-16 lands around 155 kts, so the wings are probably pretty symmetrical. Keep in mind that modern fighters fly at supersonic speeds and high altitudes and there are always going to be trade-offs.