Horse racing and other sports that require people or animals to travel from a starting point to a finishing line sometimes end in dead heats. The instances of this are quite small, especially as technology has improved through time, but Wikipedia does have a dedicated page to a "List of dead heat horse races" here. Certainty is provided because everyone knows exactly where the finish line is. Given the sheer volume of racing each year it is a probabilistic certainty that some will end in dead heats.
Offside in football is different as there is no certainty as to where a line is drawn. This depends on various parameters such as the players themselves, the position of those on the field of play, the ball, etc. Given the ever-gowning number of VAR calls on offside, one can assume with probabilistic certainty, as some point in time a "dead heat" will have to occur. In other words, the exact moment the ball is passed, both the attacker and second last defender are perfectly aligned.
I have yet to see a VAR offside decision end in dead heat. The lines always fall (at least) slightly apart. Why?
Maybe it is yet to occur. I doubt it.