The billions of dollars they’ve received is more than enough to have stockpiled a large multiple of whatever rockets Hamas was able to somehow build up in secret. “We give you $3.8B per year” should also mean “manage your own munitions when fighting a small insurgency”. If they got into another war with Iran, sure, that could warrant big brother stepping in and turning on the missile spigot, but they’re not even at war with an actual nation.