It’s not a matter of fact though, despite how they might use technology to make it look as if it is. It’s an approximation.
The cameras are not at a perfect 90 degree angle to the last defender, so there’s a need to approximate the line across the pitch based on the (approximated) position of the last defender. The refresh rate of the video means you have to approximate the precise moment when the ball was played.
The position of the attacker and last defender are determined manually by someone dragging a cursor the the point where they think the lines should be drawn (i.e. an approximation). Similarly, as football is a dynamic sport played in 3D but VAR operates in 2D, it’s an approximation as to where a player’s t-shirt line is in relation to, say, another player’s knee.
FIFA can use all the fancy graphics they like but, taken together, I reckon the margin of error could be a few centimetres. Therefore, similar with other on field decisions, VAR shouldn’t really be used to overrule an offside decision unless there is a clear and obvious error.