its not 100% accurate - we dont know exactly when the player 'kicked the ball', there is margin of error dude to camera speed etc.

Also VAR across europe uses a different tolerance

Application: Unlike some European leagues (Germany, Spain, Italy) and UEFA, which measure offside by the millimetre, the Premier League maintains this 5cm leniency, sometimes called "thicker lines".

It might well be more accurate than the human eye
but its not certain
so is it worth the hassle ?

it has highlighted the nonsense of some of the offside law though

but I think its done more bad than good