Jump drives are just plain huge

The reason they move entire planets around instead of smaller ships is because, by the time you've put all necessary equipment for a functioning jump drive into your ship, the ship's size is edging into planetary scales. ("That's no moon... it's a space station!") Putting the jump drive onto a planet is easier and cheaper than building a ship that large.

There may still be some moon-sized starhhipsstarships out there, but they would be built for very specific purposes and rare enough that the average person might question whether they actually exist or if something that outlandish is "obviously" an urban legend.