Think of it like this (and sorry if this doesn't make sense #_#)
Opponent is in the lag of a move/action otherwise, and during that small down time you're already dashing or being 'confusing' (zss holding shield in front of someone's face about to land or something is usually pretty scary), if you're grabbing in that time, even if you have 10 frame reaction speed you may have as low as 1-2 frames 'time' to actually spot dodge. While ZeRo is a monster of managing near perfect timing in stuff like this (like if there's a 10 frame window to do something he's capable of getting it out there in 1 frame reliably) in pressure that isn't so reliable.
A big part of reaction is anticipation. If you know something is coming (and ZeRo's smartness / skill of the game makes him the best or most likely of knowing) then reliably spot dodging it is more than easy. But if you're forcing someone into a guessing game or a lack of confidence then you'll get it.
So for us, the more we make people dislike shielding because of other things (well spaced bairs/nairs, ftilts in a safe manner) then their focus on avoiding the grab is going to go down and the speed of someone's reactions are getting worse for sure because of it.
As I've stated many times now. ZSS' grab being reactable requires us to anticipate/react to shield drops to "nullify" it. We can dance around people, and shields while dissipating slowly and recharging quickly can't be held forever; we also have tons of leg moves that can hit people where shields aren't covering. That's the future of ZSS mains at top level, I'm 200% certain.
In a literal sense our requirement for success is 7 frame anticipation while opponents require 13-14 frame reaction speed. But our character is so bonkers that our rewards being 2x as potent to compensate is underselling ZSS's strength A LOT.
Yeah shield drops are only 7 frames, chances of reacting to that in parity is pretty low but not humanly impossible. But if you see a shield you're certain already that it's going to be 18 frames before they can do a shield drop action; that's -a lot of room- for anticipation as it is. Majority of players have a very easy to learn/understand shield holding patterns based on what you're doing and where you are in any given moment
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As always, I refer to my Marth experience, where ultimately me being competitive with him at high level in Brawl at an international level required me timing my attacks for when people dropped their shields. I had the nonreactable fast fall into grab as an alternative to fairing to try to keep people honest, but otherwise I needed to time my **** to either be super well spaced or hitting gaps in their shield (from depletion or them doing an action) or I would lose/get punished constantly.