that's because in old power, missile ammo wasn't a thing, so it just required energy to function
and since new power, missiles have ammo, though it doesn't mean how many missiles you can hold, it's the amount of missiles a missile tube can hold/ how many missiles you can launch at once, and to improve that number, you need missile capacity modules, which can be placed anywhere on the entity, at least that was it before Quickfire, which has changed some weapon and power issues the game had for good. and it's official since last update.