Why can't you fully unlock stuff through single-player?

[Yossarian]

But you can't really use any of the unlocked stuff except in multiplayer anyway. Sure you can play a training mission with them, but that is nothing compared to an online game. It's not as if you can really do much in training mode, but the unlocks are VERY useful in MP.

I know it means you can't fly a few aircraft but other than quickly trying it out in training mode they serve no real purpose except online.