Actions in this game are constituted by what people are expecting, NOT what they see. People's reactions simple aren't that fast. ZSS does have a very slow grab, this game is about mixups.Regardless, I'm gonna disagree with you on the frame data thing, frame data tells you what situations you can use it in (in other words, to punish what, will my opponent be able to beat it on reaction if I just do it, will my opponent be able to spotdodge this on reaction, etc), that's the reason why you can't use ZSS' grabs as a threat to pressure shielding opponents like Zelda, they spotdodge it on reaction.
Honestly, not too familiar with ZSS samus so I honestly dunno what options MK would lose by simply using tactics that basically cannot be beaten by grab or d-smash, but I'd like to find out. It's probably overcompensation though, so I don't think it represents the true state of the MU.
For example you might see ZSS dash at you and expect a dashgrab and sidestep, but Zamus can reverse grab you if she predicts a sidestep. The amount of time you have to determine exactly what kind of approach ZSS is doing is tiny, and ZSS has the options of Dashgrab, reverse grab, dash attack, sidestep, and dash shield. If the ZSS player is mixing up their options the point is, you get grabbed when you read incorrectly, which as I just explained happens via "getting outplayed".
Anyway aside from the you can grab MK out of landing lag from or just by reading a Fair and reverse grabbing. If both players have equally fast reaction speed then the ZSS player should be able to react and grab (or Dsmash) which leads to grab.
This game isn't about "you should be able to", it's about what you're expecting. See my theory craft thread for details.