Life has many built in safety features to ensure successful survival of a species. Take the simplest eye-blinking when objects move rapidly towards one's face. As eye sight is a very important sensory modality for all living things, protection of the eyes via simple blinking is an involuntary reflex for almost all living animal. (Insects are exceptions, and also those that have no eyelids. I will not get too pedantic here.) So what does that have to do with Iaido? Let us ignore the blinking reflex, and imagine a sword strike targeting your head. If you have weapon or even a wooden stick in your hand, chances are, you would reflexively raise that up to block the striking sword. Your success in protecting yourself from the attack depends on many factors. Let's say you have been successful in this attempt, will you be ready for repeated blows indefinitely? If the attacker is strong and fast, you will eventually be overcomed. You might be responding emphatically, "I will strike back as soon as I find an opportunity!"
Sure, I do not doubt that, but will the opportunity presents itself before your attacker succeeds in killing you, that is another question. That is also one thing about learning martial arts, and equip yourself with skills to defend and defeat the opponent.
When we take steps to protect ourselves, most people will take a defensive posture. Stepping back at the same time to allow more space and this time to react. That is instinctively done, trained or untrained. But this also allows the attacker to advance, and continue to strike.
What if instead of stepping back while fending off a strike, you advance and push forward into
this attacker’s personal space? He will now need to step back to gain space for the next strike. At this point, the table is turned. The one striking is in a retreat while the defender is on the advance. If this continues, the defender will break the rhythm of the attack, and that is when your opportunity to strike back will present itself.
Stop thinking a block as a defensive action only, done correctly it is a move to allow a more effective counter attack.