[@1Charak2]: If we're going to insist that mystery still functions like that, the fact that there's a location in the middle of the pacific ocean that lets you summon historical and mythological figures is pretty mysterious, given not even the Association understands how it works. >.>

Servants can fire themselves, yes, but they can also [i]resist[/i] being fired. And Servants are part of Fusang's law enforcement, meaning that someone trying to escape to use a Servant as a weapon is immediately going to be attacked by other Servants. Karna can still be defeated by other Servants. So can all your other heavy-hitters.

You don't start an arms race because you literally cannot have an arms race against Servants. What do you even do to try and surpass them? There's nothing. So rather then [i]provoking the island full of superhumans that's policing itself effectively[/i], maintaining an at least neutral relationship seems like the vastly better choice.