Generally speaking, because some regulatory agency or other would refuse or revoke certification of a device that has a "disable radio" feature that doesn't work.
I wouldn't assume that a device is incapable of accumulating GPS tracking information in "airplane mode" and forwarding it when the radio is turned back on, however, or that intelligence and law enforcement agencies would never quietly disable "airplane mode" in the course of specific investigations.
Airplane mode more or less disables the phone's transmitters, but not necessarily the receivers. Even if the baseband processor is told to shut down the radio, it can still be listening for a remote backdoor message, and subsequently enable the transmitter for some nefarious purpose - similar to a remote shell.
There's a branch of philosophy that takes this to your conclusion... we can't really trust anything in this world.
However, while the NSA has surely overstepped its bounds I don't believe it is omnipotent. At some point its influence ends. I'm still willing to believe in airplane mode. Perhaps next week's scoop will change that, but for now I'm somewhat confident it works.
