Well for one thing they are kind of difficult to hit, they are falling at a very rapid speed being dropped form a ship, technically you would be better off aiming to destroy the ship rather then the titans, since the ship is probably a hittable target. The other problem is that we know you can make it so that your titans are essentially teleported down (it is one of the powers you can put on your titan), making it basically impossible to hit since it now falls pretty much instantaneous. This weapon would also have to be able to pierce through that shield that the titan drops with, and pretty much do it in one shot since i doubt it would have enough time for 2, and it would have to be extremely accurate since a weapon like that would be emplaced artillery, which means if it missed it would do a lot of damage to something you didnt want to hit. In the end there are just far better options, and it would not be worth it, on top of that we dont even seem to have weapons that can one shot a titan when it is on the ground, that is the really dumb part to me, how do we not have anything that is like artillery that deals massive damage to them?