It does exist, of course. My point is that all weapons have AoEs. You said
which is odd, as cannons, beams, and missiles all have their own version of AoE mechanics. Beams are both super accurate and have an AoE, in fact one that is in some cases better than the cannon one. Cannons use a different algorithm to calculate their area and depth of effect, but it's part of the same system. (Missiles utilize the Explosions damage system, which is different but still an AoE)
On the whole, the cannon cone doesn't really have any immense damage advantage over the beam damage model that needs to be balanced against. Essentially the big difference is, cannons' AoE is longer and thinner (and overpenetration can happen), and they can miss or hit thick armor past their threshold and just disappear, whereas beams only have a smooth scaling down with armor thickness. Cannons are just overall not as reliable for damage-dealing and taking out large chunks of stuff, though they can sometimes reach deeper if and when they do hit.