because in a sense thats absurd.
that forces the player to guess further than they can possibly react, im more than positive mostplayers cannot react to the autoguard of Zanshin, even the user or people with a DP could easily CH him out of a successful Zanshin (its doable even in CSEX)
for example were ragna to run up and 5B you 5D he reacts to the auto guard and DPs, despite the counter being successful hakumen WILL get CH'd by IDs last remaining frames because his drive has CH state near the end of his invincibility.
to remove the invincibility off the recovery would turn the risk reward almost completely out of favor for hakumen, for example (i forget which Azreal vsh aku vid it is) Azreal did a falling C on hakumen, haku 5D'd despite this being a C move (meaning its a sure fire catch) and hakumen successfully countering it, Azreal landed and blocked the counter and started his pressure.
were the invulnerablity removed hakumen would have gotten horribly PUNISHED for using his tool correctly.
even then holding D to stop the grab without some sort of safeguard (like carnage scissors) is asking to get hit (like carnage scissors) .
even then the speed the user has to react to 1. the attack hes not even 100% sure hes about to counter, 2. the attack itself and 3. the autoguard to hold D and stop a possible successful counter or a possible failure.