In regards to the actual Titanfall universe, it does and does not make sense that they wouldn't make such a weapon. The amount of resources and funding that go into Titan production, their weapons, supplies manufacturer and so on must be quite expensive alone. If they had already planned on making such a weapon, they would need a considerable amount of time and funding in order to even make a design for such. The counter to this: we do already have an ample supply of Railgun cannons, which are incredibly devastating and there are also pre-existing automated weapon systems like turrets. It wouldn't be terribly difficult to implement a larger, more destructive version. All that would be needed is a very advanced target-acquisition system that has some means of quickly pinpointing airborne ordnance moving at high velocity (such as a Titan breaking orbit) and an equally advanced firing mechanism capable of quickly preparing and launching a projectile at enough force to utterly shred a Titan into a burning mess falling down unto the planet. Then, we fall into the logic that contradicts even having such a weapon on a battlefield. It would have to be very large (yet able to function rapidly) and demand an enormous power source to accommodate its systems. What you could expect is something akin to a powerful weapons platform that would probably be anchored down. That means that if it cannot be mobile then it would have to already be built into a foundation as a beforehand precaution in case if a battle involving Titans were to occur. There would be an extremely improbable chance that such a weapon platform would be able to discern between friendly and enemy Titanfalls. Next, we have the issue of if it gets into an enemy side's hands. The last thing that the IMC would want is a weapon built to fight off the Militia getting captured and used against themselves.