And many countries operate stealth aircraft (exported F-35s and Typhoons).
Not WWII though.
Nope, but they might have if the war lasted to 1946. Countries also might have had nuke rockets in 1946, and those are in the game, and only a few countries have nuke carriers today, yet they are available in game. Also the nuke battleship never existed.
I agree with @Quasi-duck. You're thinking too deep. Radar detects units; it doesn't shoot them down. You need fighters for that. Furthermore, people don't even use secret units anymore. It would be a useless addition that no one would use.
"You can't break a man the way you break a dog, or a horse. The harder you beat a man, the taller he stands." -Jackal (Far Cry 2)
It's not that I can't use them, it's that I feel that the effort you have to put in to get a couple of any secret units isn't worth it compared to the results that you get (except nukes). Give me LTs and TBs any day,