Coutless daily car accidents tell a different story. There’s a reason that, as an average, 0.5 sec is taken as the limit for reliable human reaction time.
But that’s not the main point. There are also many instantaneous abilities in this game that you can’t react to either.
Shatter completely stuns enemies, allowing for much, much easier follow up (because well… the targets literally lie still on the ground). Even the Rein himself can get more out of it alone than a Sombra can herself out of EMP if we ignore any follow-up.
And EMP only prevents you from using your abilities and removes health from a whopping 3 out of 30 heroes. You have a mouse with a left and right click, use them.