These guys haven't even left their solar system yet, the furthest they've gone out is the inner Oort cloud, so not a whole galaxy's worth. And from what I've been reading, 1970s-era ballistic missiles don't hold a candle to things like directed energy weapons, matter obliterators, forcefields, etc.

EDIT: Also, the patrolboat carries a maximum load of 7 missiles. Even then, it's not likely to carry a full load due to weight reasons. The largest missile carrier, the dreadnaught, could probably carry about 100. Again, not a full load due to weight reasons. They still use chemical and ion engines, they have to watch their weight so they can actually move through space.