You're ignoring genre conventions. Every soft scifi movie is nonsense if you look at it from the perspective of real-life physics. Sending a single rock via unspecified FTL to attack Earth makes as much sense as the human-piloted fighter spacecraft in Star Wars. The aliens are capable of bombarding Earth across interstellar space because it makes for a good visual spectacle. Watch (or more likely read) hard scifi instead if you need everything to make logical sense.