Well bamboo is hollow, javilins were not making them heavier. Plus riding a horse is not a walk in the park, not sure if you ever have. At full gallop trying to accurately throw a javilin while holding 10 others under your other arm would be impossible. Remember there were no stirrups for the soldier to stand upon and throw, he needed to do it from a seated position. I can't imagine how difficult this must have been. I wonder if they even were able to throw javilins on the move or if they just ran around behind the enemy and threw them from a standstill.