What is risk?
I think there is a fine line between successfully aggressive and too aggressive. You want to be aggressive enough to deal maximum damage, but passive enough that you survive long enough to pick up the survival bonus (and stay alive to deal more damage). Since the level of aggression varies depending on the bots you face (very aggressive against sample bots, passive against the likes of Neuromancer, ScalarR, and Diamond), its difficult to decide how aggressive to be, and since you lose much more score in being too aggressive rather than too passive, many bots choose to go as passive as possible.
That makes sense, even if being aggressive sometimes gives you a little benefit, whenever you made a decision mistake because of unavoidable classification failures, the loss is great. So the line should be set far from danger, and even if one go beyond the line a little bit, it’s still fairly safe.
Although sometimes go beyond the line a little gives a little gain, still being fearful is the price we pay for uncertainty, and actually helps us being safe and successful after all.
In melee, being aggressive in the start of the game, gives you the benefit of 'cleaning your neighbourhood' and therefor can give you a relatively safe area when you survive till the last 4 or 5. Plus the bonus of doing a lot of damage. Then the conservative strategies kick in, like trying to never be the closest to anyone else and saving your energy.