You seem to be confusing sparring and consensual figting with self defence. I think that is where your problem lies. Sparring is useful for consensual fighting (either legally in the ring, or illegally in the street). Katas are degined for self defence, to be used pre-emptviely to attack, so the enemy never gets a go, like he would when you agree to spar/fight.
The problems with "street fighting" | Iain Abernethy
You do but the enemy (on the street) doesn't.
“The techniques of kata have their limits and were never intended to be used against an opponent in an arena or on a battlefield. They were, however, most effective against someone who had no idea of the strategy being used to counter their aggressive behaviour.” – Choki Motobu
Sparring means you have allowed the enemy the option of resisting.
A criminal assailant (mugger, rapist, whatever) has the luxury of only choosing to attack when he perceives the defender is unaware and unprepared and thus overpowering the victim without resistance. In fact, this is a primary tactic for predators.
Someone
defending against said criminal attacker will very likely not have this luxury.
Yes, an ideal outcome might be for the target to perceive the threat before an actual attack is launched and initiate his/her own pre-emptive attack to overwhelm the enemy before he realizes the fight has started or that his target is ready to defend him/herself. Unfortunately we don't live in a world where everything goes ideally.
Perhaps the assailant is the one who succeeds in landing a surprise attack. In this case the defender (if not unconscious or bleeding out) is now hurt, disoriented, off-balance, with a compromised position and structure, while having to deal with continued incoming attacks. In this case, the defender better know how to fight*.
Perhaps the defender sees the threat coming in time to avoid being sucker punched, but fails to launch a pre-emptive attack until the assault is already under way and the attacker is not incompetent at fighting. In this case, the defender better know how to fight*.
Perhaps the defender does see the threat coming and does launch a pre-emptive attack - but the assailant is tough enough and skilled enough that he is not immediately finished off or intimidated into retreat. In this case, the defender better know how to fight*.
Perhaps the defender does see the threat coming and does launch a pre-emptive attack and does drop the attacker where he stands - but the attacker has a buddy who decides to continue the assault and now won't be taken by surprise. In this case the defender better know how to fight.*
*(Or run, but retreat isn't always possible for a number of reasons.)
I'm all in favor of training to be prepared to finish an encounter quickly with a pre-emptive attack. I'm not in favor of presuming that this tactic will always work and be all that you ever need.