Excellent point but technically I don't think it's that difficult.
Assuming one frame every 1/25th of a second, the frame daylight between foot and ball can be detected and from the speed of pass the frame the foot made contact with the ball can be back calculated. Basically, the faster the pass the more recently contact with the ball was made, so the frame were contact was made can be back calculated. At which point it becomes relatively straightforward know the frame where offside can be calculated for video purposes.
Whether they can be bothered to work out the physics/applied maths behind kicking a football to get an accurate frame calculation is probably the issue.
I would suspect it will lead to Linesmen not raising their flags unless a player is offside by a couple of yards.