It is difficult. And remember in real life you don't just have one decision to make and then you get a break until the next clip. Players can run on and offside four times in ten seconds in various passing moves. At the same time we are looking out for other infringements, fouls, handballs, and who the ball came off last for corners, throwins etc, deflections, did it cross the touchline...
...plus, in the video you are placed in the perfect position to get the decision correct. In real life you are running up and down the line keeping up with play - not just stood still knowing there is an offside decision to make in the next few seconds. And you don't get 10 seconds to think about it either.