I think the theater is basically the boot camp for the actor. If you can survive the rigors of an eight-show-a-week schedule and be at your best all the time, you can handle virtually everything because no other craft requires you to get it right every single time.
There's a subtleness to camera work. You can really create intimate moments on camera, and sometimes that requires a little more precision from an actor because you have to pull people in as opposed to throwing it to them.