Spies in WW2 were destroying Dams, but to make your point you choose a ridiculously large target to make it. You can have 60 planes delivered ready for use by your nation in a matter of seconds.
IRL, who is going to admit that they are spying on another nation? Who is going to admit a spy infiltrated their nation and destroyed a nuke? Nobody.
Why don't you also complain about the fact you can build a hospital instantly?
Its an added part of gameplay and there are people moaning about how unrealistic is when the majority of the game is unrealistic. Its a simulation, it isn't going to be perfect. You can't have spies steal information on plans to war with other nations because that isnt stored in game. The missions spy can go on have to reflect in-game actions. These are in game actions that will disrupt a nation.