A cast spear is quite powerful, true, but it's not a sniper rifle. It can deliver a very powerful aimed shot at very short range, or an unaimed volley at relatively short range. Javelins were mostly used in volleys around 20 meters. Plus they need a wind-up movement.
That's akin to the looters's rock issue, most thrown weapons are excessively accurate with excessive terminal velocity and too fast casting.
That's akin to the looters's rock issue, most thrown weapons are excessively accurate with excessive terminal velocity and too fast casting.