It's just physics, just an example, say the guy on the horse is going 20mph, the guy weight 150lbs. You try to pull the guy off the horse when you are standing still. Think of trying to stop 150lbs travel 20mph away from you and you manage to pull him to a DEAD STOP with distance of less than a few feet travel. What kind of force you have to assert by your grip? Remember the law of physics. Calculate the force it takes.Are we all kind of making that up though?
The guy might fall off the horse, so is your spear that got pull off your hands. You lost your weapon. When the next guy on the horse comes, you are chopped meat!!!!
As a Chinese grew up in Hong Kong, do NOT trust too much what you hear.