It's generally not that the attacks by themselves are unreactable, but keeping an eye out for them while you're busy looking out for a number of other kinds of attacks too.
If someone just stood in front of me and said "I'm going to throw you" then it would be easy to react to. But if they were doing heavy stagger pressure, throwing in crossups and overheads, and then suddenly went for a throw, it would take a moment to even realize what happened, if they did it right.