They aren't?
Actually, in a way, they are. If you think back to the talk of Kerry selecting a VP candidate, you may remember talk of the FBI vetting Kerry's short list. This is SOP, more in order to find embarrasing skeletons than to award a clearance...but vetting, all the same.
As others have said, election to POTUS brings automatic clearance. As does election to the Senate, but Reagan's dictum of "Trust but Verify" is closely followed.